GPS works fine for me. Usually I get a fix within seconds. Sometimes it takes a bit longer and sometimes SatStat says that I’m already connected to a few Satelites but I still don’t have a fix. All in all my worst TTFF (time to first fix) was about 1 minute.
Advanced mode is missing for me too, but I think it’s already like this since FP OS 1.2.8
I hardly ever use location, and if, then only in situations where I need no play store (i.e. somewhere outdoors).
Though, whenever turning on GPS, play store crashes.
If you didn’t change the system in any other way (any other flashed ZIP, nor any root-requesting app changing things on /system) you can unzip the system image from the official ZIP and fastboot flash system system.img. This preserve your data because userdata is not touched at all.
If you did modify the system, then that wouldn’t be easy. Most Google Apps are invisible to the user, meaning they do not show on the launcher, and trying to manually find and remove them with some root-enabled app is quite arduous. I followed once this blogpost, but I think the package format in OpenGApps isn’t the same, so you have to engineer something. Try to search the Web for another method or re-flash system.img and apply your changes again if that’s possible.