This seems pretty sketchy.
the app store description is pretty clear on what it actually sends to google, and it's actually less than what most keyboards send - swype, the standard google keyboard on android, and i think most others send everything up to their respective servers so they can sync personalized predictions across devices, this keyboard only sends the queries that you explicitly ask google to search for.
You have to either trust that google isn't flat-out lying, or try to MITM and analyze the traffic using something like wireshark, which would be difficult assuming it's properly encrypted.
I don't care about gifs or emojis. One of the only things I miss on iOS as an ~8-year user of Android is the swiping keyboard. This gets you that.
Additionally, I think the privacy of using this keyboard without "full access" is better than using the android builtin keyboard. Unless google is doing some side-channel attack (is this possible? seems harder than on android) to funnel your keystrokes out through some other google app on your phone which does have internet access, it's just a dumb keyboard.
Now, the sketchiness of guiding users to turn on "allow full access" by default... well. It would be paternalistic for me (as an armchair privacy enthusiast) to say "google shouldn't be encouraging the general population to divulge their keystrokes".
It should probably not surprise you that the authors of your keyboard software are theoretically capable of seeing what you type on your keyboard though, if you think about it.
User @colinbartlett above says Google explicitly states what is sent, however. So am I right that it looks like we are trusting Google with what they say they are sending? I'm ok with that, I think.