I've never used the City Mini GT but I'm not sure why it wouldn't fit unless that stroller's basket is super tiny. You may need to deflate the Bubble Bum. It stores fairly compactly.
* I just looked up the stroller and you probably could even fit the Bubble Bum deflated in the pouch in the back of the stroller. The only issue I see with the basket is that it appears to fairly open on the sides at the top - meaning fearing that something might fall out if you put too much in the basket.