So, to use Gboard's undo button, you can type something first, then tap the four squares button in the top-left corner. This will replace all the autocorrect suggestions with the undo button.

Along with ↩️↩️ RedoGboard adds Undo and Redo buttons for easier editing. Undo can reverse per-letter deletion. Redo is great for undoing accidental Undo. Available with Gboard version 15.