Well, I think it is quite okay to say this kind of words.
Mainly because this is a group discussion and is testing your speaking ability.
If you have said ,'gotta' or 'gonna', I sure that the teachers can understand the meaning of it, they are somekind the nature of English! I suppose in England or America will not so tradition that say 'got to' or 'want to'.
I think this can show your nature of English and it is not an bad idea.
By the way, you need to remind that do not use CHINESE english. For example, when the group member and you talk at the same time, you should say "AFTER YOU" but not "YOU FIRST".