Summarize:

Here you can access all installed keyboards. Any of the settings here that match those in the Gboard will not apply to Gboard. Those must be changed from the Gboard app to affect Gboard’s behavior. Go to “General > Keyboard” to access the keyboard options. This button displays a list of all usable keyboards. Tap “Edit” and tap and drag Gboard to the top of the list. Release and press “Done” to save your settings. This moves Gboard to the top of the list when switching between keyboards. Go back to keyboard settings and tap “Text Replacement”. Here you can set filters and shortcuts for when you type. Tap the “+” button to enter a phrase and its replacement and tap “Save” to complete. For example, iOS defaults with “omw” to be automatically replaced with “On my way!”. Changes to this setting will also be made in the Gboard app.
Open iPhone or iPad settings. Access keyboard settings. Tap Keyboards. Set Gboard as the main keyboard. Edit text replacements.