Gemini mosters need no tributes in order to use their effect. It does, however, use your normal-summon for that turn. It's eff. is treated as a normal-summon, so you cannot use the effect in the turn it is summoned. For example, if I were to tribute-summon Gigaplant, I could not use it's effect during the turn. During my next turn, however, I could have Gigaplant's ability activate instead of normal-summoning a monster. I hope this helps!
EDIT: Ninja'd by Kuuga.
EDIT2: Hang on, Kuuga. I always thought that using gemini abilities counted as a normal-summon?