I downloaded the initial version of GBoard today for iOS and was looking forward to using it. Unfortunately, I had to stop after a minute because it kept misregistering characters I was typing, and I don't have that problem with the iOS built-in keyboard.
At first I thought this was due to the layout being different, but it isn't. Here's a screenshot to compare GBoard with the iOS keyboard:
(Difference provided by Resemble.js.)
My thought now is that GBoard must not register keys in the same way that iOS's keyboard does, leading to a lot of mistyped characters for me, and this might be due to having a different text size setting in iOS.