I've never been one of them people that used to look up to Gimr as the staple of G&W gameplay. In my opinion Songun is the best G&W player but it's easily possible that Regi is actually better. I honestly cannot tell you where Gimr gained that amount of credit from. It's never been a big secret that MDVA isn't exactly the strongest region out there so even if he had decent results it wouldn't mean all that much.
How do you define "decent"? Even if Diddy and ZSS are only about -1 that's still an additional two losing matchups among the top tier characters. That may be decent if taken out of context but if you add it up with his even worse matchups against Rosalina and Fox it's honestly not looking too good. I take it we can agree that Sheik is a losing matchup as well? I think it's fair to say that G&W, despite having a couple of decent matchups up there, just doesn't have the overall spread required to be better than mid tier. Shulk also has a ton of 'decent' matchups but even the most optimistic people would not rank him higher than mid tier. I think that's where G&W belongs as well.
I define decent as that:
Let's say we have a Top G&W, he's in pools, his next match is vs an alright Diddy Player.
He will most likely win despite the matchup not being in his favor.
Let's say he reaches Top 32, he now has to fight a Top Diddy.
In this case, he is very likely to lose unless matchup inexperience comes into play, which could help him. But overall, his chances of winning vs that player are still pretty low.
But, with the -2/-1.5 matchups that are Rosalina or Sonic, he's likely to lose to a player of that character who's alright but has indeed worse skill than him, even if said player doesn't know the matchup.
I think you took my counter argument as a response as to why G&W can possibly place in a major w/out secondary backup:
Let me get to the point, despite Regi & Songun's bringing a few results to our character & showing how good he can be in the right hands, as you mentionned, some of his matchups are extremely rough & may prevent you to advance any further in a tournament because of some kind of random has beaten you with what's basically your worst matchup.
However, i do think Game&Watch holds a strong niche in the metagame, with a bunch of solid matchups going for him. And as i said, going in with a secondary for these rough matchups is the way to go if you want to place.
Tl;dr , Game&Watch is most likely in the lower end of Mid Tier, with the niches he holds & few placements here & there, i don't think he can go too low, but yeah, he's not like, exceptional. He's your typical mixed bag character, with flaws & pros.
I really wish he had a SH AC on either Bair or Fair, that would be amazing.