After editing an image, it still looks the same in the gallery

All edits you do on an image won't touch the original image, but are stored in a sidefile (raw images) or the exif data (jpg images). The edits are just getting applied when you export the images.

If you always want to see the edits you've done, you can enable this by going into Preferences -> Displaying -> Render XMP settings.

Note: This will reduce the performance of browsing and viewing significantly, so only use this if you actually need it.