Impossible to homemake gaff cards (printing ink wise)
Sure you can split cards, but you cant make anything like those Ultragaff? decks or gaff card decks with all the werid looking cards from a printer. Ink will always smear and rub off. Impossible to keep the ink on
Depends on what you are doing. You can recreate any passive gaff with card splitting and a little care. You are probably right about active gaffs but I find those cards less powerful. The only effect using an active gaff that really hits the spectator between the eyes (IMHO) is Twilight Angels. That's because Paul Harris realized that the best way to sell the effect, the best way to make the spectator truly believe magic just happened, is to have the card signed and given away.
I hesitate to give the OP a specific list of cards I have gaffed since it might inadvertently tip the method for some of my favorite effects. I certainly make double backers for every deck I own that doesn't have one commercially available.