Gotta leave this one up to execution. Goh battling a Legendary is like Goh battling Ash, and it’s gonna be real hard to take this seriously.
Especially when you consider that at no point in the entire series has Goh ever been realistically at a level to take on legendaries, in skill as a trainer, his Pokemon's strength and self-confidence. The Articuno episode only further proves this.
Every one of his direct encounters so far where he somehow stands on an equal footing with them has had bullshit writing
(Zapdos at full health goes down to one weak Blaze Kick), plot convenience
(SwSh plot with Wolves weakening Eternatus and him being the one to Max the Pokeball) or bent worldbuilding to favor him
(Suicune, let's not even bother with this debate, it's an objectively bad setup).
I can see Gary being able to capture one if he brings out all his big guns, he could potentially be at the level to take down at least one with some effort.