Gmail (and other messaging platforms) have a “learning” capability that identify your frequently used words or terms that are not in their dictionary and will “autocomplete” in certain circumstances and correct misspellings to conform with your preferred usage. Generally, though, you need to “opt-in” and turn on these features but perhaps you have done so or your other preferences roll in those features.
Google remember your previous manually typed words. When you type something similar in the future, it’ll suggest your custom database of words, even if it isn’t an official English word.