Follow the steps on How to import registration fees (includes sample data file) and when preparing the spreadsheet, include the gift information you are attaching to the registration on just one line as follows in the example below:
 
REGImpIDREGFeeUnitRegFeeNumUnitsRegFeeGiftAmtRegFeeRcptAmtGFImpIDRegFeeAppliedAmt
00001-587-0000000325Individual1251500001-545-0000002051525
00001-587-0000000325Corporate Sponsor1500350  
In the example above, the same Participant Import ID is listed on two rows as are the different units that person is registering for, however, in order to link both to the gift, we will need to include the Gift Import ID and Applied Amount on just one line. To do this, total the applied amount from the amounts in previous columns and include it on one line.