The carb and sugar content on the nutritional label does not tell you what the GI of a food is. Its a bit more complicated that that. Generally though, foods that are lower in sugar and carbs are lower GI. You can find a GI list of foods at Mendosa's site. This link doesn't go directly to the GI list, but there is a link to it from it...
http://www.mendosa.com/
Yes, you can use a little low fat mayo on tuna, or eggs. If you are looking for recipes and ideas, you might want to check out Sugarbusters, and the forum on here. Sugarbusters is based on the GI, and there are a lot of recipes and information on the related forum on here. Here is a link to the main area:
http://www.3fatchicks.com/forum/forumdisplay.php?f=85
I hope this helps!