Yes that is a large difference. What batch size are you using? When you are using 2 GPUs the batch is split onto the different GPUs which can cause a problem if the network contains batch norm layers.
If you lower the batch size you can try with one GPU to see if that works with the 128x128 image before moving on to two GPUs.
Other than that, GANs have had problems with high resolution images before. Perhaps you can train the network on 64x64, save the weights and then retrain those weights on 128x128?
I’d also recommend some sort of system to check the loss + gradients to see that the network doesn’t suffer from gradient explosion. That can cause some very bad images. Have a look at tensorboard(x) and visdom.
Goood luck