Guess, this is my change to help you out as opposed to the misused-English-words-thread where you give me hints on improving my language skills.
It has different meanings - and I guess this is confusing some of you guys.
Kind of everything:
like: I like it --> Ich habe es gern (or maybe I would use here "Ich mag es" or even "Ich mag es gern").
ok: You're welcome / my pleasure --> Gern geschehen. Habe ich gerne gemacht.
please: That would be your "Kebab gern" example.
mag: See also my like example. But keep in mind, that "gern" is not a verb. So either you ask "Hast du es gern?" or "Magst du es?"
"Kebab gern" is short for "I beg your pardon, but would you mind preparing me a Kebab? - just kidding...
I would say (a) and (b). (c) does not really apply, you might just make a full sentence out of it which would be "Ich hätte gerne einen Kebab" - then it is absolutely correct (but not really used in spoken language). But "Kebab gern" is more of a Swiss German expression; maybe in Austria I would rather say "Einen Kebab bitte" (but alsy Kebab gern is accepted and not offending at all).
Not sure, if I got your context 100% right, but it rather means "You're welcome" or maybe "I take care of it" or even your "ok/sure" makes sense.
For the "Isch do no frei" I would rather answer with a simple "Ja". To answer with "gern" the question should be "Darf ich mich hier hinsetzen?" (even though this question might not be very common... sorry for this example).
This is short for "Bitte, gern geschehen", so just a bit longer, maybe a glimpse politer for "Gern geschehen" or even "Gern(e)".
This works. Not in all contexts. So a "Kebab sehr gern" does not really work, but "Das habe ich sehr gerne" is just like a comparison.
Again depending on the context. So "Das habe ich nicht gern" works, but "Keinen Kebab gern" does not make sense at all.
Ok, I wrote a lot... and I am still not really sure about how to explain all the "gern"-options. But hope to give you some hints... maybe I meet some of you at the Wednesday drink this week - then feel free to discuss about it.
Finally I recommend to have a look at the examples in my preferred online en-de dictionary: http://dict.leo.org/?=gern