Most gallery apps do not delete photos straight away. They first go to a Recently Deleted, Trash or Recycle bin folder.

Step 1: Check the trash of your gallery

Samsung Gallery: Menu → Recycle bin (about 30 days).
Google Photos: Library → Trash (60 days for backed-up items, 30 days for others).
Xiaomi Gallery: Albums → Recently deleted.

Open the photo and tap Restore.

Step 2: Gone from the trash too? Scan the phone

Open Deeps (Deleted Video Recovery) and allow access to files.
Run a Deep or Full scan.
Preview the results and restore the photos you need.

Do it soon: once the trash is emptied, new files can overwrite the deleted photos. Avoid taking new photos or installing apps until you have scanned.