Can my phone use the watch GPS?

I don't believe so. You can however export your Gaia routes to Garmin which will allow you to follow your Gaia routes via the map display on your watch: 

If you're looking at exporting your Garmin walks into Gaia:

www.reddit.com/.../

Oh and just a note, phones don't lose GPS when out of cell range. Rather, they lose the ability to dynamically pull map tiles if you haven't already downloaded them (meaning you'll see a blank spot on the map or no map at all). It sounds like you might want to download offline maps for the area you're thinking of hiking in before heading out. This way you'll always have a map even if you lose cell connection.

Personally though, I just export my Gaia and AllTrails routes onto my watch. It means you never need to pull out your phone. Your watch battery will almost always outlast your phone's, and youve still got a fully charged phone in case of emergency if you're not using it constantly for navigation.

Phones use A-GPS, so they use cellular to help more quickly pinpoint your location. 

when in airplane mode, and perhaps in less than ideal satellite view, the phone fails to accurately locate your position. And it can take uncomfortably long to lock on, if at all. I am well aware that otherwise GPS is a receive-only signal feature

exporting routes from Gaia is too clunky a flow given the frequency and variety of trails/routes I do. If there was a background/automatic sync (like how your activities sync to Strava without “exporting” to Strava), then that would be better at least. 

ALSO while battery life is a consideration, the readability of the screen on my iPhone is incomparable to my tiny 7s, especially when the goal is to look at the location, forks, features, etc and make decisions off that  

it would SEEM the Garmin explore app actually solves my problem though, I think. Not as detailed mapping options as Gaia, but in a worst case I can use the location and copy it across to Gaia. 

Perhaps check out AllTrails if you're not too strongly tied to Gaia. It has a native export to Garmin feature from within the app which allows for that super quick workflow.

I one click export to Garmin, sync the route to the watch and the mostly navigate using the watch. As you said, I'll pull out the phone to look at more complex topo features and then put the phone away. It's an easy process when you've got the same route showing on both devices.

I do a far amount of mapping and orienteering racing (so off trail in heavy woods), some using a GPS based 'punching' application.  In general, my Fenix will provide a slightly better track, in part due to the dual band dual freq capability, but I have rarely ever seen my phone have any serious difficulty with location.  With others using this or a similar app, when they have issues it is generally a configuration problem such as not allowing the application to run in the background or when the phone goes to sleep. in this case, every time the phone is woken up, it has to reacquire location.  This might be something to check if you have not already. This fits in somewhat with the A-GPS discussion that, technically, helps more quickly determine your location but this is only on startup to update the GPS almanac and ephermerides - really the same thing the watch does when synced getting a CPE/EPO.  Having said that, phones also use other sources of data (cell tower, wifi networks etc) to help with figuring out rough location (hence even when indoors with not satellite view it can still figure out where you are (or close).

Just to add some details to this (and only slightly related), an iPhone can certainly use an external Bluetooth GPS such as the Garmin Glo 2, sadly, as noted, the Garmin watches do no broadcast location.  The thing about an iPhone is that application can only connect to the iPhone location service not the actual device - and the iPhone decides which device to use for location - the attached or internal (Android has the ability to let you select).  For example, with the Garmin Glo connected, the only way I can tell if the phone is using it is by using another GPS app that shows the GPS data, specifically the rate of the locations (iPhone is 1/sec while the Glo can provide 4/sec - up to 10 for Android).  Note that I am talking about the more generic situation where you would want the watch to provide GPS to the iPhone which was available to any application such as Gaia.

I agree to razmichael. If your phone looses GPS, there must be an issue with your phone. Before I got a Fenix, I was navigating using an old phone that did not even have a SIM card installed, and there were zero GPS losses.