Still unclear. Aren't you staying in Gruyeres? How will you get to Charmey to start your walk, and at what time?
Since you are staying in Gruyers and returning to Gruyeres, why not leave bags at your hotel. I checked the station listing, and I cannot find evidence of lockers. That is quite surprising.
Broc does not seem to have lockers, either at Village or Fabrique. Also a bit surprising.
Bulle has left luggage with reasonable hours:
but does not seem to have lockers.
It is possible to take the train to Bulle, leave luggage there, and with some delay, take a bus to Charmey. See attached screen grab.
Getting luggage on the way out would involve some kind of detour via Bulle, with only hourly service by train.
As an option, I'd also (If i had time, which I won't for a week or so,) contact a tourist information office in the region, by searching for Gruyers and Broc or other locations and ask a real person about lockers and there suggestions for your problem. Please be clear about where you intend to start and finish..
While I wait for that answer, because I want to double check your route for altitude changes and times, there is another possibility. Possibly you could send bags from GVA to Montreux upon your arrival and take only light baggage to the Gruyeres region? Regular service is 12CHF per piece. Express adds a flat 30 CHF flat fee to the 12CHF....possibly one fee for all bags, but i can't tell from the listing.
Ordinary service would not get your bags there until the morning after you arrive in Montreux,but express service, admittedly expensive, would get it to Montreux by 6 Pm on the day of your hike.
Or, if you route through Montreux, on the way to Gruyeres, you could drop bags there yourself.That could be quite a scenic trip.
Here are some links to the SBB luggage transport services.
www.sbb.ch/content/dam /sbb/de/pdf/bahnhof-services/dienstleistun gen/gepaeck/Stationen_ Reisegepaeck_Express_D FIE.pdf
I've looked at the maps:
map.geo.admin.ch/?topic=ech&lang=en&bgLa yer=ch.swisstopo.pixel karte-farbe&layers=ch.bav.ha ltestellen-oev,ch.swisstopo.swiss tlm3d-wanderwege&layers_visi bility=false,true&X=16 1495.30&Y=574937.47&zo om=7
map.wanderland.ch/?lang=de&route=all&bgLa yer=pk&resolution=1.5& X=578924&Y=163083&laye rs=Wanderland
That looks like a really nice walk. Depending on where you start in Charmey, you might have a bit of of descent to loosen up your knees. Overall, about 12 km, and not too much elevation change. See link for altitude profile: