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<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2021-08-11T13:36:49.6868099+00:00 -->
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<!-- Copyright (c) 2021 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="gmtx_OneTimeNonRefundableUpfrontPayment" name="OneTimeNonRefundableUpfrontPayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AdditionalConsiderationUponAchievementOfCertainDevelopmentAndCommercialMilestones" name="AdditionalConsiderationUponAchievementOfCertainDevelopmentAndCommercialMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_RoyaltyPaymentsPercentageOfNetProductSalesIfCommercializationAchieved" name="RoyaltyPaymentsPercentageOfNetProductSalesIfCommercializationAchieved" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gmtx_CommitmentsAndContingenciesDetailsLineItems" name="CommitmentsAndContingenciesDetailsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_CommitmentsAndContingenciesDetailsTable" name="CommitmentsAndContingenciesDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_TwoThousandAndSeventeenLicenseAgreementMember" name="TwoThousandAndSeventeenLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_TwoThousandAndEighteenLicenseAgreementMember" name="TwoThousandAndEighteenLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_TwoThousandAndNineteenLicenseAgreementMember" name="TwoThousandAndNineteenLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_TwoThousandAndEighteenMasterLicenseAgreementMember" name="TwoThousandAndEighteenMasterLicenseAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_TwoThousandAndNineteenSaleAgreementMember" name="TwoThousandAndNineteenSaleAgreementMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_PaymentOfOneTimeNonRefundableNonCreditableInitialLicenseFee" name="PaymentOfOneTimeNonRefundableNonCreditableInitialLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AnnualNonRefundableNonCreditableDevelopmentFeePayment" name="AnnualNonRefundableNonCreditableDevelopmentFeePayment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AdditionalConsiderationOfContingentUponFutureCommercialization" name="AdditionalConsiderationOfContingentUponFutureCommercialization" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_PaymentForUpfrontLicenseFee" name="PaymentForUpfrontLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_PaymentForAnnualLicenseFee" name="PaymentForAnnualLicenseFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AdditionalConsiderationUponAchievementOfCertainDevelopmentMilestones" name="AdditionalConsiderationUponAchievementOfCertainDevelopmentMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AdditionalConsiderationUponAchievementOfCertainCommercialMilestones" name="AdditionalConsiderationUponAchievementOfCertainCommercialMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_AdditionalConsiderationUponAchievementOfCertainSalesMilestones" name="AdditionalConsiderationUponAchievementOfCertainSalesMilestones" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_OneTimeMilestonePayments" name="OneTimeMilestonePayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_ContingentAnnualFeeUponCommercializationPercentageOfNetSales" name="ContingentAnnualFeeUponCommercializationPercentageOfNetSales" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gmtx_ContingentAnnualFeeUponCommercializationAmountOfNetSales" name="ContingentAnnualFeeUponCommercializationAmountOfNetSales" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gmtx_ConditionUponCertainMilestonePaymentsWaivedPercentage" name="ConditionUponCertainMilestonePaymentsWaivedPercentage" type="num:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gmtx_ContingentAnnualFeeUponCommercializationDescription" name="ContingentAnnualFeeUponCommercializationDescription" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gmtx_RelatedPartyTransactionsDetailsLineItems" name="RelatedPartyTransactionsDetailsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_RelatedPartyTransactionsDetailsTable" name="RelatedPartyTransactionsDetailsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gmtx_ReductionToAdditionalPaidInCapitalMember" name="ReductionToAdditionalPaidInCapitalMember" type="nonnum:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
</xsd:schema>
