Standalone GPS simulators only made sense when arbitrary sig gens were not capable of playing back continuous stream with 3-5MHz BW.
The problem with old sims is that they are either single channel, single GNSS or just a few sats constellation capable. They are difficult to set up and are really basic.
These days almost any SDR-like siggen (or tx-capable SDR) can do it and the only limit is your imagination and software coding skills.
I also badly needed one for production testing and, like you, unsuccessfully tried to buy an old dedicated sim - than someone recommended
gps-sdr-sim. I have used it as a test tool and never regretted not buying the "real thing."
You can do some stupid navigation scenarios if you want (like flying 200km above ASL, etc.)
If you are really serious about getting the real thing or need traceable certification - ask you local R&S sales rep.
Leo