Never installed an access panel that wasn't a lid or door. If we have to screw it off is it code for an access panel?
See other question for example: What can I use for a very small access panel?
They just installed a mega automatic transfer switch in my basement for my emergency generator. The face of it (the access panel) is screwed in with four knurled screws. This is the only access to the interior, including the dedicated breakers. I know it's code for electrical.
I do not know if there is a standard code that covers gas line access, probably model building codes, but I don't see how it is very different.
Code mavens (that's French for experts)?
Yes, it is acceptable; you just need access. However, you do need to make sure that the thing that it covers is to code as well.
For example, if you have a standard box, you can splice inside of it and put a blank cover on it. You can't, however, just have a splice behind drywall with a hole that is covered by a blank plate.