Long-press and slide
Long-press a → à á â ä æ ã å ā. Long-press n → . Long-press $ → the full currency row (€ £ ¥ ₹ ₩). Long-press ? → ¿, . → • …, - → – —. If popups don't appear: Gboard Settings → Preferences → enable “Long-press for symbols”.
The second symbols page
?123 → =\< reveals © ® ™ ¶ § ± ÷ and friends that have no long-press. Long-pressing keys here goes deeper still (& → §).
Personal dictionary shortcuts
Gboard Settings → Dictionary → Personal dictionary → + : word “Zürich”, shortcut “zrh”. Faster than any popup for your dozen repeat words. Syncs across your Android devices on the same account.
Samsung Keyboard differences
Samsung's board also long-presses for accents, but the symbols live under !#1 with a swipeable second row, and text shortcuts hide under Smart typing → Text shortcuts. Or install Gboard from the Play Store and follow this guide verbatim.
Add the language itself
Gboard Settings → Languages → Add Español/Français/Deutsch. Spacebar-swipe switches; predictions and corrections follow the language — the actual fix for “autocorrect destroys my Spanish”. Related: autocorrect changing symbols.