If the front caliper is below the rear reservoir, I'm almost positive it is, it'll work. It's not as far below as the front calipers to front reservoir so it'll be slower, but will still work.
Some will say that gravity bleeding doesn't work and that you need pressure or vacuum, but I've been solo bleeding systems using only gravity for a couple decades.
Another trick that's handy (especially with LBS and ABS where you REALLY don't want to deal with air intrusion) if you need to remove a hose or caliper, depress the master cylinder piston a little bit and it'll keep the rest of the system from going dry. On a car that means using a bar or something to hold the pedal down a bit. On a bike I use a zip tie to hold the front and a screwdriver or other slim thing to hold the rear pedal down, slip it between the pedal and stop screw or whatever limits the up-travel of the pedal. You don't need to depress the piston enough to make pressure, but if you do it's no biggie, when you first crack the bleeder you'll get a tiny spurt then nothing. In most masters the piston only has to move about 1/8" to block the port, but the lever/pedal travel will be much more than that since they all have a leverage advantage. I usually move it enough to feel a little pressure and that's a guarantee that the port is blocked. I've left systems open for days using this trick and the master was still full. Once the repair is done open the bleed nipple, a beer, then release the caliper piston. Pour fluids as needed, just know that beer isn't good for brakes and DOT4 might fuck with your taste buds so try to not mix them up.