Is a flat the same as g sharp on viola?

Yes, F-flat and G-sharp are the same note on the viola. They are enharmonic equivalents, meaning that they sound the same but are written differently. This is because the viola is tuned in fifths, like the violin, and there is no note between F and G. So, if you want to play a G-sharp on the viola, you would finger it as an F-flat.