Hankal wrote:
What's there to comment? It has to be in an IFR-approved installation
to be used for IFR.

It is an IFR installation.
Is a 337 required?

I think so.

I am not an Avionics mechanic, so don't beat up on me.

I'm not either, but I did have an IFR GPS installed once a few years ago. There
was a Supplemental POH that needed to be filed with my airplane's POH. That
supplement has to be approved by OK City. I think the 337 is the mechanism for
getting the approval. The SPOH spells out the limitations on using the GPS for
IFR, so it can't be used IFR until that supplement is in effect. It will say
things like the requirement for having a current database in order to use it for
instrument approaches, for example.