herbuhduhflergen

On paper its moveset doesn't look great, but in practice, being a bulky Grass type with access to Earthquake, Horn Leech and Bulk Up is amazing in certain scenarios, and the nature of Reborn lets off-picks like Gogoat thrive. It's a one trick pony for sure but it does its job very, very well. It sets itself apart from Torterra and Chesnaught with its better survivability, having no 4x weakness and carrying a STAB life-leeching move, so there isn't really a direct "replacement" for it; that being said you'll still probably want to seriously consider dropping it around the end of the Agate arc.

after being before  flying gym and pick up for  tera?

Yeah, I'd probably drop it by Ciel. Most opponents after that are too strong for it to get going and keep going, even in seemingly advantageous matchups like Amaria or Hardy. It certainly won't be your mainstay because there are some battles it just won't do much in - Cain, Radomus, Aya etc. Know what you can set up on and Gogoat can do wonders, bring it into the wrong battle and it's pretty much useless. It's far from impossible to work into a 6 'mon team (I've done it twice) but it does excel more on a rotation.