First off, I had to Google to find out just what the heck is a "G" license. Turns out it's an Ontario thing. When I grew up in Ontario, the usual start was a 90-day "Learner's License". Of course, times have changed.
I am writing here to point out that "Canada" has nothing to say about driver's licenses. Parliament cannot pass legislation in relation to that matter. It's constitutionally assigned to the provinces and provincial legislation varies. There are no "Canadian" rules.
So it is wrong to say that "Canada does not allow you to have two driver's licenses at the same time." Moreover, I fail to see how any province could impose such a ban. Makes no sense. I have long held licences for British Columbia, California and the Philippines, having owned a home in each location at the same time. I double dare British Columbia to tell me that I cannot maintain or use my California license when home in LA, or my Phils license at home there. The comment about "even if they let you keep it" makes meme think any provincial government would have to wrestle by CA or Phils licenses from my cold dead hands. I would absolutely refuse to surrender them.
However, I agree with the rather obvious statement that, if you have an Ontario license and get stopped while driving, of your you should produce your Ontario license. If, at that point, you pull out a foreign license, you are an idiot.