Basically, if it's required for certification, then it has to either be serviceable or covered by an MEL. No MEL means that if it's required for certification, then it has to be serviceable.
The real question, is what is required for certification??? Look at the basic aircraft flight manual. Note that this is the basic flight manual. Any STCs would obviously modify this.
If it has a caution panel diagram, or a table that lists caution panel segments, then those have to be serviceable.
If you have a fuel gage in the flight manual, and it's unserviceable, you shouldn't dispatch.
You may be able to get a temporary flight permit from your local regulatory authority, but that's a different matter.
The danger if you dispatch with these things unserviceable, and you have an accident, you can be (rightly) accused of not being within the bounds of your certificate of airworthiness, and hence your insurance is invalid...
And where do they teach you this??? Beats me. I had to learn it, and indirectly at that, while working as a certification test pilot.