Hey there,
by Gary Oak Edition I assume you're talking about "Gary`s mod). I don't think it will be discontinued but it will take some time.
As for your actual question: you can use NSE Sprite Editor to change sprites. Other useful tools you might need would be AdvanceMap and A-Trainer. I haven't done ROM hacking in a long time but this should be your starting point.
Otherwise you will find help in the hacking section here on PC.