How do you type the ₿ Bitcoin Symbol in Android Google Keyboard?
Gboard settings (cogwheel on keyboard) > Dictionary > Personal Dictionary > (your language) > + button

For the shortcut box, add whatever symbol or word you want that will make the symbol appear on the autocorrect list in the moddle. Copy and paste the symbol for the shortcut field.

Yours may vary. Razer Phone 2, Android 9, Gboard v9.1.8