A lot of MODs include comment ("// MOD xyz BEGIN", "//MOD xyz END") lines when changing phpBB core files, so you can identify changes made by a MOD without running extra compare tools.
It is very helpful if you have more than one MOD installed

Is it possible to have the same behaviour with gallery?

should be quite easy if you read the code.
I mostly use

Code: Select all

if (class_exists('phpbb_gallery_integration'))
{
// some other code here
}

At least I won't force users to edit all files to add the comments, so I will only do that when I change the code anyway.