This is SO weird - and really annoying! But here's how I "fixed" it. When the aggravating little "1" pops up as a "reminder", go ahead and swipe it so it takes you into that part of Setup. You'll probably see that your Samsung keyboard is already chosen. HOWEVER, what makes it stop nagging you is to turn on the "Show Keyboard button" selection at the bottom. It then puts a little keyboard icon at the bottom of your screen and turns off the annoying "1" at the top. You're stuck with it. If you turn that selection off, the "notification" starts to pop up again. Trust me, the little keyboard at the bottom is a lot less annoying than the pop-up at the top. I couldn't find any other way to get rid of it. Hope this works for you.
Here's an even better way that I found "out there:"
It's not because you're using Gboard, it pops up even if you are using Samsung keyboard. It is just there to help for faster keyboard changing who like to change frequently. You can disable the notifications by long press on that notification, then tap on details, then scroll down to Virtual Keyboard and then tap on the blue button to turn it off. once it's off you won't see the notifications again