Garry's Mod Steam Gift Problem (HELP)?

Hi everyone , i recieved a gift from my friend and its garrys mod but when i press ''open my gift'' , it says i need source sdk base 2006 , so i downloaded it from tools , BUT it didnt work i still get the same error so i downloaded source sdk base 2007 and still it didnt work ... what do i need? my friend told me he wasn gonna gift me another game to make it work such as HL2 but he can help me buy it if i pay him so i dont really want to spend my money, and if i download HL2 , not buying. Will i be able to recieve the gift????? ( Please tell me what games will work if he gift me them to make me able to recieve garrys mod. ) and i am not sure if he gift hl2, i recieve it will i be able to recieve Gmod??? same to other games that work with gmod.. please help me.