Add and remove custom words from the Gboard iOS keyboard dictionary
Adding custom words is done by typing them once. Once typed once, the word will be available using Glide Typing. It may not have a high priority, but gliding accurately enough will choose it, and the more it is used the higher priority it becomes just like any other word.

Removing an individual custom word is not possible currently. You can clear the entire custom dictionary in Gboard &rarr; Keyboard Settings &rarr; Clear my dictionary.