What are some anime you can think of that are neither mainstream nor a hidden gem?

not exactly sure what counts as mainstream, but this (and most of the anime ever realeased)
TONIKAWA Over The Moon For You

nhk ni youkoso

This is kinda similar to the neither underrated nor overrated. My answer would be;
MUSHISHI

And possibly, many seasonal shows. They are neither of them.

Date a live

GTO Great Teacher Onizuka

Kinos Journey
Yokohama Kaidashi Kikou
Sayonara Zetsubou Sensei
Kubikiri Cycle The Blue Savant and the Nonsense User

monogatari in a way
it isn’t mainstream, i have friends who watch anime but have never heard of the series when i say it’s my favourite
and it isn’t a hidden gem, cause it’s quite popular among more ‘hardcore’ anime fans…?

i agree with Gintama

anilist.co/anime/20

Very strange selection OP. How do you decide if a show is mainstream or not?
The way I do it is to check the number of users who have either watched the show or are planning to watch it. Given this criterion, Monster and Sangatsu no Lion are definitely mainstream. (You could check the numbers by visiting the anilist pages). Gintama is almost mainstream esp in Japan. LotGH is almost mainstream too.
Ping Pong seems to be the only fit.

A more difficult question is: what's a hidden gem?
They come in at least three types:

1. good anime from >10-15 years ago from the present time that has been "forgotten" by many (i.e. less ppl recommending the show to new blood)
2. sophisticated anime that enjoyed a small cult following but was largely misunderstood/dismissed by the masses
3. anime having themes and genre affiliations that keep it from being accessible to the general public

Moreover, hidden gems could be gems purely due to either their cinematography or animation or plot. If you're lucky it could be a gem for multiple factors.

But the big question remains:
How to know if a show is a hidden gem?

Here's one way: Again this show needs to be on very few users' anime lists. If a mainstream anime is on 500,000 users' anime lists, a hidden gem could have as few as 1,000 users. Next, you would need some proof that the show is a gem. That is, a review that appreciates the show.

So what are anime that are neither mainstream nor hidden gems?

For one these could definitely be seasonal anime from recent years that received moderate success. Moderate success either because they were controversial (think Yesterday wo Uttate) or maybe they just couldn't leave a solid impression. Quantitatively we might be looking for these:

1. Number of users who have completed the show/plan to watch it ~ 100,000 at max
2. Mixed reviews
3. on users' PTW list for a long time

So maybe a show like Dance in the Vampire Bund is neither mainstream or a hidden gem.

Or we could have anime that has a large number of positive reviews, people know they're good, but they nevertheless don't watch it.