Just to correct your conclusion, the texture doesn't have to be in its regular size. The resolution can be for instance 1024x1024, 640x256, etc.
Let's dive into the problem though. Wasn't your game running while using GGMM? It must be closed. Also try to doubleclick the metals in TXDWorkshop and untick compressed, save the TXD,
replace it with the nrg in game (gta3.img) and save it.
This helping through mobile sucks ass, I'd fix your problem much faster on my PC.
However, I hope this was at least a bit helpful
Cheers.