Welcome to the forum - I see you've just joined. Don't remember seeing your Mom's name here, but it's nice that she recommended us. I've only done one GOG, and didn't like the mess I made of composition so tossed it out. I have seen on this site many beautiful ones (ADDISUE and CALAMITY, to name a couple) and those w/experience w/give you advice. I just wanted to welcome you and say we're anxious to see your work.

Hello and welcome!
Depending on your personal preference and where the mosaic will be will determine the adhesive. Good old weldbond works fine but may take some time to dry clear under large pieces and should not be used in a moist area or outdoors.
Some like MAC glue as it dries very clear and quick, again-not for outdoors.
regular silicone adhesive caulk works well, it is not crystal clear there is a slight cloudiness. this bothers some people, I dont mind it.
Then here is Lexel a crystal clear silicone. a little more expensive but no cloudiness at all.
It depends on your preference in glues-I like silicone for the thickness and instant grab yet able to move things around for a short time. Its also easier with the silicones to make sure you have the entire back of the piece covered to help prevent grout bleed. But I know others who like to work with weldbond or mac.

Wackyweeder,
I am wondering why you say Mac Glue is not for outdoors? I have been using it for my GOG windows and have them hanging on my covered deck with no problems whatsoever. I like it the best because it dries crystal clear.

Lin-was linlee here did a test of mac and weldbond etc-glued up stuff and submerged it. The mac did turn cloudy like weldbond with immersion. On a covered deck probly not a problem. The thread is here somewhere, maybe it still archived I will see if I can find it.
Its still probly better than weldbond tho.....

OK-I went and found it-tons of good info i this thread for lots of different uses and adhesives.
Far down in the thread she did the immersion test and said that the exposed MAC scraped off easily with a fingernail but that it was stuck tight to the glass. I thought I recalled more about that but it may have been on another forum. Anyway-tons of people say they use weldbond in birdbaths etc with no problem also. So maybe it has to do with how well you are sticking things down or particular climate or whatever. for a window on a porch or something its probly not an issue, for a birdbath or flowerpot I personally wouldnt do it.
Anyway theres the thread......

Here is a link that might be useful: adhesive test