If you use the keyboard for desktop mode:

1. Open system settings
2. Navigate to "Input Devices", "Keyboard"
3. Go to the "Layout" tab
4. "Configure Layouts", then "Add"
5. Add "German (US)"
6. Switch the layout on the "Taskbar" on the bottom right of the screen.

You can then use Umlaute like on any German keyboard, plus on the "a,U,o" keys themselves when you press AltGr (right alt). Any Sonderzeichen are like in the American layout. If you want to change that just use the normal German layout instead of German (US).

There is no way to change the layout of physical keyboards in the gaming mode. Those settings don't change anything in the gaming mode and but the gaming mode settings only work for the on screen keyboard.

Bonus Umlaut of the day: Ö

A friend gifted me a laptop with US layout. One day I figured I really needed those German characters. I did the same as @okiloki@feddit.de, except I ended up picking the US-International layout, and removed the old layout since I didn't need it anymore.

Put your keyboard to "US international with dead keys", then use quote (") and the letter, you'll get an umlaut.

Enter desktop mode. Go to the system settings -> Input devices -> Keyboards and click on the Layouts tab in the top. Check off the box next to Configure layouts then click the Add button to continue. Search for German, click on Ok and then on apply. Hope this helps!

How are the possibilities to change the on screen keyboard?

If you talk about a physical keyboard that is plugged into your steam deck: have a look at eurkey

Steam Decks run Linux. (The specific DE is KDE Plasma I think.) So you can find answers by searching for "Linux" if searching for "Steam Deck" doesn't get results.

One way is to enable the "Compose" key which lets you enter special characters or sequences by typing switches of more commonly-available characters. I think the Steamdeck OS has a setting for this; but I don't have one so I can't check.

For letters with umlauts you press (and release) Compose, then type a double quote (need to hold shift for this part), then type a vowel.

For reference Wikipedia has a list of Common Compose Combinations

Alternatively if you can map an AltGr key I've read you can type umlauts by typing AltGr+[ and then typing a vowel. There might be a setting for this too.

my guess is that you have a custom keyboard with qmk firmware (why else use ANSI layout?). so pick a mofifier key in VIA and make it switch the keyboard to a different layer, then maybe set the A O U and maybe S keys to Ä Ö Ü and ß. if you're not running QMK, just buy a DE layout keyboard for god's sake.