What do you do with GHB units when you aren't interested in them?

It is usually better to keep them for skill inheritance if you aren't interested in using them. Some of them have skills that you may want to use on other units (such as Brazen Atk/Spd from Linus or Fortify Dragons from M! Kana). Sending them home is generally not worth it because they are limited units. You can get feathers from sending any hero home. You can only get skills from these units or from another hero who is most likely in the 5* pool.

I just keep some for fodder like Linus, Walmart etc.

If they don't have anything worth foddering, then I save them in case they ever get a weapon refinement.

Absolutely nothing. They just sit there

GHB units are limited and I'm determined to keep at least one copy of each of them. The ones I'm interested in get promoted right away while the ones I'm less interested in sit in Barracks limbo while I decide when to promote them.

I keep them for "Free to Play" guide videos for Infernal Maps, like GHB, BHB, or LHB i.e. Infernal L! Lucina map. I have used Narcian and Ursula a lot this way, and just recently used Canas & Finn to help clear L! Lucina's Infernal map. XDDD ^w^

Also, some of the "not good" ones are some of my best team units i.e. Xander, Camus, and Michaelis. I plan on Walhart being a part of one of my Horse Emblem teams eventually. And I am really looking forward to F Takumi's rerun cause I was taking a hiatus the first time he ran.

Upgrade one of them to five stars, keep the rest for a rainy day.

I used a couple for skill food early on but better units with better skills have come along. Wasn't a good long-term strategy.

All the above. I don't see the point of keeping characters who will just be sitting there useless.

You hold on to all copies of limited-copy units. Because they are all sacred and should be used to make those units as strong as they could possibly be with merges.

That's just me tho. I respect anyone with a free unit that's merged to their highest potential.

If I get a GHB unit and I am not interested in them, I'll definitely send home the 3* unless if it has something good (like Grani on a 3* Walhart). If the 4* only has trash skills, it's sent home.