Only, when you use the entire word, "sympathique", the conotation is not the same.
Roughly put, "sympa" is the most used form to imply that someone (or something, like a party, a place, clothing...) is nice, cool, friendly, etc. Also, depending on the tone you use, it can also mean that something was "so-so", "not too bad", especially if someone asks how much you liked something:
"Comment t'as trouvé le film?" (What did you think of the movie?)
"Sympa..." (Cool / not bad,...). If you really enjoyed, loved the movie, you wouldn't use "sympa".
On the other hand, "sympathique" would sound either more formal meaning the same as "sympa" (which is clearly informal), for instance talking about your colleagues in a formal context ; or sound a bit ironic (often used with a "Ah" of surprise, disgust: "Ah..., c'est sympathique...!").
Note that the noun is used entirely, "la sympathie", and can mean both what you feel about someone who's nice to you; and also, like in English, the fact that you share someone 's feelings, mostly sadness, grief,... ("Toute ma sympathie" = "Toutes mes condoléances", roughly, about someone's death), which is the original sense of the word (from Greek, "to suffer with").
Well AFAIK "un" is pronounced more like an (ah-n), so I'd say yes. (S-un-pa) for you would be the equivalent of sampa or sanpa for native English speakers (the m sound gets swallowed a bit so it works either way IMO)
Its hard to spell out phoenetically because the French "un" is more of a nasal sound that isn't present in English (at least not that I can think of)
"Gentil" has more the conotation of "kind", "sweet', "well-behaved", "polite" ; whereas "sympa" is the most common equivalent to "nice", "friendly", "cool", when generally speaking about a person.
Besides, used about an adult, "gentil" is nowadays a bit derogatory, at least used like that:
Cette fille est gentille = That girl is kind as in "but she doesn't have much personality / is not very interesting / is ugly,..."
Cette fille est super gentille! = That girl is really kind as in "she's so sweet, she's generous, her intentions are always good, she's genuinely helpful"
As often, the tone you use will also show what you mean ("Que penses-tu de cette fille?" - "Oh...bah, elle est gentille..." : clearly you don't find that girl interesting, maybe even boring)
Used about a kid, it will rather mean that he/she is well-behaved, obeying, etc. You say "Gentil !" to your pet or your kid to mean "Behave !'.
Finally, you use "gentil" ONLY about people or their actions, while you can use "sympa" for people, their actions, objects, events,... When used about things other than people, "sympa" will often mean "so-so / good but not great / just OK...":
Someone brings you a gift, you can say to them "C'est gentil!' = you really mean it's kind of them.
Someome brings something to your party, you can say "C'est sympa!" also = you rather mean that what they brought is cool, or that it's great that they thought of that
Someome asks what you think of a party, or a film, a place, etc., then you can say "C'est sympa..." but NOT "c'est gentil" = it will mean that it's good, cool,... but not awesome.
Please see my comment above. It's a very common adjective, used to imply:
- someone/-thing is nice, cool, friendly, etc.
- someone/-thing is "eye candy", e.g. "Wow, regarde cette fille/ce mec (look at that girl/guy), sympa!!!", could be about a dress, a car, a landscape, music (then it'd be "ear candy"! lol)
- something is not too bad, to express your degree of appreciation; let's say, between "nul" (it sucks!) and "génial!" (great, awesome)