Alpha Gumoss is a level-11 field boss around (-113, -628) in the Sea Breeze Archipelago. The nearby Sea Breeze Archipelago Watchtower is around (-168, -663), southwest of the encounter. Keep the two coordinates separate when planning your approach.
Head northeast from the watchtower
From the watchtower, use a northeast bearing toward the Alpha. Your X coordinate should increase toward -113 and your Y coordinate toward -628. Follow passable ground and recheck both numbers after a detour; matching only one can still leave you far from the boss.
Look for the level-11 Alpha when you reach the area. Ordinary Gumoss also spawn elsewhere, so seeing a small Gumoss along the way does not mean you have found this encounter. The Pal location finder can help you compare it with other fixed targets.
A flower Gumoss is a different goal
Gumoss also has a flower form, but visiting this Alpha spot does not guarantee you will find one.
If you are collecting that form, check the appearance of the Pal you encounter before spending your supplies. Do not treat this boss route as a way to force a flower Gumoss to spawn, or assume a particular time of day will guarantee one.
Use small hits near the end
Begin with an attack whose damage you can judge. A weapon that felt weak against a tougher boss may remove much more of this Alpha's health than you expect.
If your Pal is dealing most of the damage, recall it before the final throws. Keep your aim on the intended Alpha rather than a nearby ordinary Gumoss, and give yourself time to check the sphere's displayed capture chance.
After a failed throw, look at the remaining health before adding another hit. More damage is not always the best next step, especially when your next attack could end the encounter.
Bring spare spheres, and use the displayed capture chance to decide whether to switch to a stronger type. If your equipment is not ready, knowing the route makes it easy to plan a return trip.
Take one on a wood-gathering trip
Gumoss's current partner effect helps with logging and reduces wood weight while it is in the party. This effect does not stack, so extra Gumoss will not multiply that same benefit.
Try one on a normal gathering trip and compare the cargo you can carry with your usual team. You can keep the other slots for travel and combat instead of filling them with duplicate support.
For another fixed encounter in the region, Alpha Sweepa is south of Small Cove and adds a group of smaller enemies to manage.