Hi, we just replaced a bathroom window because it was in the tub/shower and had wood around it so you could imagine the black mold that appeared after a few years of showering. We replaced with glass block and it looks great and is flush with the shower wall... the problem is on the outside of my house it is not flush with the siding and leaves about 3-4 inches in as if it was pushed in, what do we put to fill in the gap all the way around? The frame on the siding around that window is still in tact. Hope I explained it good... if not I could always take a pic... thanks for any help!

Just some wood trim, such as cedar, or whatever matches your house. Not knowing how much moisture might get around the edges from the shower, condense when it hits cold air, and rot, you might want to make them easily replaceable.

Cellular PVC might be a better choice if you have painted trim. If your trim is white you won't have to paint it. Cellular PVC (AZEK is one brand name) doesn't rot. Be sure the top flange of the "window" (sounds like a block unit) is flashed properly.