Yes, it's a GC. I will ask more questions about who actually does the measuring and ordering of the door. I don't want to end up in a situation where the vendor says the contractor measured wrong, and the contractor says the vendor fabricated it incorrectly.

The glass company should do glass doors. That means templating and installing. Which means your GC puts a markup on all the materials and labor, to supervise and integrate everything.

The problem is that you have gotten a "quote" from a GC which isn't really a quote.

A bid for a job involving multiple aspects like a bathroom remodel would include all the specifications for each aspect of the job.

You have mentioned shower enclosures. You would provide the GC with exactly" what you want - it would specific the thickness of the glass and the design of the enclosure. Any special elements such as Showerguard for the glass would be specified.

If the contract doesn't contain the specifications how would the parties determine whether the work had been done "properly".

Also to some extent the design and finishes are going to impact the cost - for example, specific designs of tile or even certain tiles might be more expensive to install which impacts the labor cost on a real bid for a job.

In response to your actual question, when I remodeled my bathroom, the I supplied specifications for the shower including the enclosure. The GC provided the glass company who actually installed the glass as well as mirror I had ordered.

with our new build our builder had the shower door/ glass company come in to do all measurements, get our decisions on style and hardware and handle the installation. Another builder we interviewed wouldn’t install shower glass/doors. Go figure. Said they had “too many problems with it”

Thanks, everybody. I do have a detailed cost/quote from the GC, which includes measuring for and installing the shower door; I'm responsible for providing the door. I will either have him remove this from his job entirely and handle it wtih a vendor I choose after the rest of the work is done, or have him assume responsibility for ordering the door to my spec, based on his measurements, and have him install it. I won't split up the measuring/fabricating/installing.

Make sure that he installs blocking at the wall attachment points. You cannot attach heavy glass to just drywall!

Personally (and I may be biased) I would have a glass shop make and install a all glass hower enclosure. I would not trust a GC or an off the shelf system, typically adjustable which looks awful in my opinion.