There is also racht (which nobody else mentioned), which is essentially a larger version of gagh. I'm not sure if any in universe details are given on their relation. The only time I recall it being seen was in DS9's episode Melora, as well as when Jadzia Dax ordered it in DS9's Playing God:
As for gagh, expanding on the other answers here: There are many varieties. I'm not sure if any other types are detailed in universe, but from the DS9 episode Prodigal Daughter which I just coincidentally happened to watch last night, there is at least:
- Torgud Gagh, these wiggle on the way down, compared to:
- Filden Gagh, these squirm on the way down, compared to:
- Meshta Gagh, these jump on the way down
- Bithool Gagh, these have feet
- Wistan Gagh, these are packed in targ blood (targs are a commonly referenced animal native to the Klingon home world, conceptually similar to a wild boar).
The dialog in that episode implies that there are at least 51 varieties:
KIRA: How much gagh did she order?
ODO: Fifty one cases.
EZRI: Each containing a different variety.
KIRA: There are varieties of gagh?
EZRI: Oh, yes. I can remember what each one tastes like, and the way they feel when you swallow them. Torgud gagh wiggles. Filden gagh squirms. Meshta gagh jumps.
BASHIR: Are you all right?
EZRI: Bithool gagh has feet. Flush it out the airlock. All of it.
ODO: Environmental regulations.
KIRA: Why don't you just give it to Martok?
EZRI: He'd insist on sharing it with me as a point of honour. Wistan gagh is packed in targ blood. I have to go now. See you later.
I do not claim to know the difference between "squirm" and "wiggle", but those were the words Ezri Dax used to describe them.
I also know that Klingon's prefer lively, squirming gagh and racht - complaints along the lines of "this gagh is barely moving, it's half dead" are often seen. I also recall an episode of TNG -- A Matter of Honor -- where Riker sampled stewed gagh on the Enterprise, only to be surprised by the relative liveliness of fresh gagh later served to him on a Klingon ship:
RIKER: A Klingon delicacy. That's heart of targ -- this is pipius claw. And this is stewed "gagh."
RIKER: Serpent worms... would you like to know how they're prepared?
... later, on a Klingon ship ...
RIKER: This is "gagh," isn't it?
KLAG: Very good. You did some research on our nutritional choices.
RIKER: Yes... but this is still moving.
KLAG: "Gagh" is always best when served live. (covering smile) Would you like something "easier"?
KLAG: If Klingon food is too strong for you, perhaps I could get one of the females to breast feed you.