@Dawn Ryan yeah this is a real and very plausible culprit — 15mm is close enough that if there's any switching regulator noise on the GNSS module radiating into the 2.4GHz band, your BLE receiver is going to see it, and you're right that at those ranges you have almost no margin to give up.
The first thing I'd do is a quick sanity check: power the GNSS module off completely and run your range test again. If your link quality recovers noticeably, you've found your problem and you can start isolating whether it's conducted noise on the supply rails or radiated from the GNSS board itself. If there's no difference, it's probably something else — antenna orientation, ground plane issues, whatever — and you can rule this out.
If it is the GNSS, a few things that tend to help: a small copper pour ground shield between the two modules makes a meaningful difference, and making sure they're on separate LDO-regulated supply rails rather than sharing a switcher is probably the biggest single fix. Conducted noise from a shared switcher into the BLE module's supply can degrade receiver sensitivity just as badly as radiated interference, sometimes worse.
Also worth checking — do you have a ground plane break or any slot in the PCB between the two modules? That can actually make radiated coupling worse by forcing return currents to route in ways that create unintentional loop antennas right between your two modules.
