[SOLVED] No Game Mode in Windows 11 ?

Go to Start> Type Game Mode Hit enter. You should see a new window show up with the option to enable or disable game Mode for your GPU, unless you're referring to the Game Bar.

Also, no, I'm on 24H2, 26100.4349 and Game Mode wasn't removed.
 
EDIT: Solved by modifying a registry entry:

Windows Registry Editor Version 5.00

; 🔧 Riattiva Game DVR (registrazione automatica)
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\GameDVR]
"AppCaptureEnabled"=dword:00000001
"GameDVR_Enabled"=dword:00000001

; 🔧 Riattiva GameDVR anche in GameConfigStore
[HKEY_CURRENT_USER\System\GameConfigStore]
"GameDVR_Enabled"=dword:00000001

; 🔧 Riattiva overlay Game Bar e l’apertura automatica
[HKEY_CURRENT_USER\Software\Microsoft\GameBar]
"AutoGameModeEnabled"=dword:00000001
"ShowStartupPanel"=dword:00000001
"UseNexusForGameBarEnabled"=dword:00000001

; 🔐 Permette l'avvio con controller per tutti gli utenti (richiede privilegi admin)
[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\GameDVR]
"AllowAutoGameMode"=dword:00000001
"AllowGameDVR"=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\GameBar]
"AutoOpenWithController"=dword:00000001