I got my Nexus 5 to pair with Gear Fit 1.49 and do the firmware update. I sort of did it accidentally after thinking I had screwed up the Fit, so I'm not sure if any of this was unnecessary. Here's what I did:
Installed 1.49 and tried to pair. This connected initially and popped up the confirmation number, but connection timed out after that point.
Installed GearFitManager-1.apk from post #11 of the other thread. This listed itself as v1.15
Installed GearFit-update.apk from the 1st post of the other thread this was the one from the 9/4/14 link. Version is now 1.25
This is where I though I had messed it up. Tried to pair, it gave me the BT confirmation number and then went into a firware update. Fit changed into BT update mode. Update stopped at 4% of the file copy and stayed there. Resetting the Fit always brought it back into update mode, there didn't seem to be any way to cancel it. On the Nexus there was also no way to cancel the update. I restarted BT and rebooted the phone. As soon as you go back into the Fit manager it detects the Fit and starts the update process, which never got more than a few percent done. Sometimes it would pop up the BT code on the Nexus, but never on the Fit. The Fit still stayed in update mode.
Installed the GR.GearManagerStub.apk. Didn't try to pair again after this point
Installed the 1.49 apk I had originally tried again. Opened up the Fit manager, it found the Fit and popped up the code, once again it automatically went into the update, but this time it completed. The Fit restarted itself after the update and the Nexus paired to it immediately. Most all the functions seem to be working.