<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>117 - Disclosure - DEBT</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>eol_PE712480--1210-K0004_STD_366_20121231_0</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001366246</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2012-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2012-12-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="12 Months Ended" /><Label Key="Calendar" Id="1" Label="Dec. 31, 2012" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>4</Level><ElementName>us-gaap_LongTermDebtTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="eol_PE712480--1210-K0004_STD_366_20121231_0" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;
&lt;p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;NOTE&amp;#xA0;8&amp;#xA0;&amp;#x2014;&amp;#xA0;DEBT&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; MARGIN-BOTTOM: 0px; MARGIN-LEFT: 2%"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;MIG
Notes&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;In December
2007, the Company acquired MIG to accelerate its presence in China.
In December 2008, the Company amended the MIG merger agreement to
acknowledge the full achievement of the earnout milestones and at
the same time entered into secured promissory notes in the
aggregate principal amount of $20,000 payable to the former MIG
shareholders (the &amp;#x201C;Earnout Notes&amp;#x201D;) as full satisfaction
of the MIG earnout. The Earnout Notes required that the Company pay
off the remaining principal and interest in installments. In
December 2008, the Company also entered into secured promissory
notes in the aggregate principal amount of $5,000 payable to two
former shareholders of MIG (the &amp;#x201C;Special Bonus Notes&amp;#x201D;)
as full satisfaction of the special bonus provisions of their
employment agreements. The Company had fully repaid both the
Earnout Notes and Special Bonus Notes as of March&amp;#xA0;31,
2011.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px; MARGIN-LEFT: 2%"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;Credit
Facility&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;In December
2008, the Company entered into a revolving credit facility (the
&amp;#x201C;Credit Facility&amp;#x201D;), the terms of the Credit Facility
were amended in August 2009,&amp;#xA0;February 2010,&amp;#xA0;March 2010
and February 2011. The Credit Facility, as amended, provided for
borrowings of up to $8.0 million, subject to a borrowing base equal
to 80% of the Company&amp;#x2019;s eligible accounts receivable. The
Credit Facility expired on June&amp;#xA0;30, 2011 and all borrowings
were repaid in full.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for long-term debt.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 210

 -SubTopic 10

 -Section S99

 -Paragraph 1

 -Subparagraph (SX 210.5-02.22)

 -URI http://asc.fasb.org/extlink&amp;oid=6877327&amp;loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher SEC

 -Name Regulation S-X (SX)

 -Number 210

 -Section 02

 -Paragraph 22

 -Article 5



</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>DEBT</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>DEBT</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.glu.com/taxonomy/role/NotesToFinancialStatementsLongTermDebtTextBlock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>1</NumberOfRows></InstanceReport>
