If you’re getting good daylight during the day, a 10W panel should be more than enough to recharge the internal battery during the day.
My wife pairs hers with a iP8 and she reports no problems with dropping the connection, so I suspect that’s not an issue if your software and firmware are up to date on both the phone and GTM.
I haven’t measured the outputs of the panels I’ve been using. They’re Voltaics. The 6W one is rated at 930 mA peak amperage. I also have several units that use a 2W and 3.5W panel together, which they rate at 890 mA peak combined.
I acquired a couple of 18 volt panels from China for free due to a shipping error, but had to acquire charge controllers to use them with my battery packs. These had readouts that would allow you to peak the output via MMPT, so can provide volt and amp readouts. On recent days with the low winter sun they were showing output of about 300 mA. I have no stated numbers of the rated output to compare that to, but it’s taking a significant drop from what you’d likely expect as a peak output. Always good to have plenty of panel when working on installs in this seasonally gloomy part of the planet.
I have some 10W panels from China I’m now testing, but no numbers on them yet.
IIRC, the GTM is rated for operation down to -4 F. That’s a battery-imposed limit, as LiPo’s lose capacity at cooler operating temps. At zero C (assuming you’re using that since you’re asking from Vancouver) you’re already taking a hit, but go down to 20 F/-7 C and things are getting iffy fast. That’s probably less of an issue in by-the-sea Vancouver than here in chillier Urbana, though.