Today's slang : ガチで=Gachi de
ガチ(gachi) is an abbreviation form of ガチンコ(=Gachinko)
It means 本気で(=honki de) / 真剣に(=shinken ni) / マジに (=maji ni) : (to do something) seriously / to get real/for realt/ playing something for keeps

Ex,1)ガチで勝負する(=Gachi de shoubu suru) fight with the gloves off=真剣勝負(=shinken shoubu)
Ex. 2) ガチで勝負しよ！(=Gachi de shoubu shiyo!) Let's play game seriously.
Ex. 3) ガチで泣けた(=gachi de naketa) It made me cry badly.

なんか、ガチでやってる？(=Nanka gachi de yatteru?)