I have an iphone XS, and have been using GBoard since I got the phone a year ago with no problem.
Recently I upgraded to ios 13, and I've noticed GBoard having issues ever since.
Frequently when I open an app, gboard does not appear, and instead the default english keyboard appears (I have gboard set as primary). When I click the change keyboard button, I have to cycle through all my keyboards multiple times before gboard actually shows up. The first few cycles, gboard will appear for a split second before crashing and going back to the default english keyboard again.
Previously, I had an iphone 7S, which had the exact same symptom. GBoard was working perfectly until I did an OS upgrade, and then it started crashing the exact same way. It gradually got worse with each OS upgrade until it became completely unusable.