<?xml version="1.0" encoding="us-ascii"?>
<!-- DFIN ActiveDisclosure(SM) XBRL Schema Document - http://www.dfinsolutions.com/ -->
<!-- Creation Date      : 2022-03-10T21:13:16.2842465+00:00 -->
<!-- Version            : 5.0.1.728 -->
<!-- Package ID         : bccf0333da4a4aa6a6341ade2771a311 -->
<!-- Copyright (c) 2022 Donnelley Financial Solutions, Inc. All Rights Reserved. -->
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  <xsd:element id="gray_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" name="TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gray_GainLossOnChangeInFairValueOfPreferredStockTrancheObligation" name="GainLossOnChangeInFairValueOfPreferredStockTrancheObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gray_LiabilitiesCurrentMember" name="LiabilitiesCurrentMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_SeriesCPreferredSeriesBPreferredAndSeriesATwoPreferredMember" name="SeriesCPreferredSeriesBPreferredAndSeriesATwoPreferredMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_SeriesATwoPreferredSeriesBPreferredAndSeriesCPreferredMember" name="SeriesATwoPreferredSeriesBPreferredAndSeriesCPreferredMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_DividendsRightsAmountPerShareOnOutstandingSharePayable" name="DividendsRightsAmountPerShareOnOutstandingSharePayable" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="gray_PercentageOfAggregateNumberOfSharesOfCommonStockOutstandingOnLastDayOfPrecedingYearAddedToPlan" name="PercentageOfAggregateNumberOfSharesOfCommonStockOutstandingOnLastDayOfPrecedingYearAddedToPlan" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gray_TwoThousandAndTwentyEquityIncentivePlanMember" name="TwoThousandAndTwentyEquityIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_TwoThousandAndTwentyEmployeeStockPurchasePlanMember" name="TwoThousandAndTwentyEmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_InducementGrantsMember" name="InducementGrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_CommonStockAdditionalAggregateNumberOfSharesReserved" name="CommonStockAdditionalAggregateNumberOfSharesReserved" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="gray_NumberOfNewlyHiredEmployees" name="NumberOfNewlyHiredEmployees" type="xbrli:integerItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" />
  <xsd:element id="gray_EffectiveIncomeTaxRateReconciliationMarkToMarketGainOrLoss" name="EffectiveIncomeTaxRateReconciliationMarkToMarketGainOrLoss" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gray_IncomeTaxesAbstract" name="IncomeTaxesAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_DeferredIncomeTaxLiabilitiesDepreciation" name="DeferredIncomeTaxLiabilitiesDepreciation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gray_IncomeTaxesLineItems" name="IncomeTaxesLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_IncomeTaxesTable" name="IncomeTaxesTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true" />
  <xsd:element id="gray_OperatingLossCarryforwardsExpirationBeginningYear" name="OperatingLossCarryforwardsExpirationBeginningYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gray_TaxCreditCarryforwardExpirationBeginningYear" name="TaxCreditCarryforwardExpirationBeginningYear" type="xbrli:gYearItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" />
  <xsd:element id="gray_UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" name="UnrecognizedTaxBenefitsIncreaseDecreaseResultingFromPriorPeriodTaxPositions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit" />
  <xsd:element id="gray_UnrecognizedTaxBenefitsIncomeTaxAccruedInterestAndPenaltiesExpense" name="UnrecognizedTaxBenefitsIncomeTaxAccruedInterestAndPenaltiesExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit" />
</xsd:schema>
