you don't have to remove the wallboard. you need to put cement board on top of the wallboard then place your glass tiles on top of that. I wish you the best of success!! PS: you can get step by step instructions you type in "howtoinstall tbacksplash tiles on U-tube
If your using a somewhat clear glass tile, you need to seal the wallboard using a water proofing paint designed for just this very reason. A good quality tile supplier can set you up with what you need.
This paint will seal your wall fine. Water proofing is a great idea, but realistically your not spraying the walls like you would a shower. So the paint sealer is all that is needed.
Be sure to choose a color that will not make the glass look bad. Also the material that you choose to fasten the tile to the wall makes a big difference in the look of the tile once its been placed on the wall. You do not want a dark gray thinset on a clear tile. A white thinset is normally used. But the thinset used does not hide the wall behind it. This is why you would paint the wall with a color blended match to the tile so it looks professionally done.