Albeit it might seem attractive, Gboard is not really a helpful keyboard selection, though at times it may. For one if you want glide typing, that is still available on Samsung Keyboard. All you need to do is go into keyboard settings. Second, Samsung Keyboard offers more versatility and efficiency. This is proven by the numbers column on top of the default letters keyboard. No need to switch between the numbers and letters keyboard. In my opinion, I would definitely stick with Samsung Keyboard because of these features and because it is native to Samsung, and compiles with Dark Mode. Now the rest is up to you 😉🍁🍁🍁🍁
GBoard also follows dark mode, though I have to say that Samsung does do it better plus it supports automatic switching. In every other aspect though GBoard runs circles around the Samsung keyboard. Samsung does not have multilingual input, has awful autocorrect that actually gets worse over time, and has a propensity to lag.
Gboard DOES have the number row like Samsung keyboard, "glide typing" and also has auto language detection so no need to manually switch languages. I agree with @dejanh, gboard runs circles around Samsung keyboard, auto language detection being the main reason for me personally.
It's faster for people to either tap the word properly or to use Gboard. Swiping each time (multiple times!) to show new words instead of being better at guessing the next work is neither convenient or fast. Gboard's got the upper hand