Noted. I'm unable to reproduce at the moment. What were the exact settings you used on the field (name, description, field options, default value, order, defines sort order, etc...)?
I set the codename as RANDOM so it's auto-generated by Composr but beyond that there were no other changes.
https://compo.sr/docs/tut-fields.htm#title__9
I did as you indicated on my test v10 install... custom Gallery field of type codename with RANDOM default value. But I'm not getting any errors / it seems to be working as expected.
Did you explicitly name the field "_gallery"? Maybe this is another bug that was fixed in master but not yet released.
I didn't name the field "_gallery", I will try to reproduce the error once I get some time and will reopen if I do, but as you cannot reproduce it from what I said then I imagine it was "user error" somehow.
https://compo.sr/docs/tut-fields.htm#title__9
Did you explicitly name the field "_gallery"? Maybe this is another bug that was fixed in master but not yet released.