I usually work on higher bit image types. Actually, all the time, I haven’t messed with GIF’s in a long time. GIF’s are an indexed color pallet of 256 colors or less, so they are flat also, no layers. Apparently, Photoshop partially locks them. I don’t know what that means really because it behaves almost like fully locked. If other sites are telling you to just click on the lock in the layer to unlock it, you’ll never make it Sydney.
To enable the ability to unlock and fully edit, the “Color Mode” needs to be changed from Indexed to anything higher.
Menu: Image > Mode > Pick a higher color mode type like RGB.
Last note: This is in Adobe Photoshop 2020 at the time of this post.