I just got off the phone with the Co-Creator/Co-Owner of the Wizard and Nestabilities. The Craft Foil that Spellbinders sells (
www.spellbinders.us) to be used in with their Dies and Wizard is 38 ga. However, there is a disclaimer that some other manufacture craft foils might have a tighter tinsil strength, which then the foil would only bend and not cut.
I have used Amaco Craft Foils (not heavy gage) and have cut pop/beer cans and even the disposable aluminum pans with my Spellbinders Dies and Wizard. I have also used the Craft Foils that Spellbinders sells on their website. All with good results.
I would suggest playing with what you have and have fun.
Hope this helps.
Kar