I have an iPhone XS and have been using GBoard since I got the phone a year ago without any problems.
Recently I upgraded to ios 13 and found that GBoard has had problems ever since.
When I open an app, I often do not see a blackboard, and instead I see the default English keyboard (I've set the board as the primary keyboard). When I click on the keyboard change button, I have to go through all my keyboards several times before gboard is actually displayed. For the first few cycles, gboard will appear for a fraction of a second before it crashes and returns to the English default keyboard.
Previously, I had an iPhone 7S that had exactly the same symptom. GBoard worked fine until I upgraded my operating system, and then it crashed the same way. With each operating system upgrade, it got worse and worse until it became completely unusable.