Ci vediamo domani basically means "see you tomorrow" http://www.wordreference.com/enit/see%20you%20tomorrow
No, we wouldn't be penalized if we translated this sentence into future tense especially when there is an element referring to near future "domani". I would just slightly alter the word order to match that of the Italian sentence : "We will see each other tomorrow at the gym."
The present tense is often used to express what will happen in the future. If another element in the sentence refers to the future, the present tense can be used. http://italian.about.com/od/verbs/a/italian-verbs-present-tense.htm