Yes the grav gun was great. But if I remember correctly Far Cry had some fun physics too: pushing barrels onto guards, making boats out of pontoons, and general Fun With Dead Bodies (TM) - ranging from kicking them off ledges and down stairs to watching them bob up and down in the water, leaking out their guts
The slick way you could switch from first to third person perspective for the vehicles worked so well in FarCry - and the rush you got first time in the hang-glider, and the variety of vehicles. Sorry - they had HL2 beat in that department.
Granted, FarCry had some rubbish levels and didn't have the consistant story or dramatic sweep of HL2. But it did have those huge open-ended levels and great sense of freedom and exploration missing from HL2.
I'd say they were both great games - but I still say FarCry stole alot of HL2's thunder by being first.