Gemini Monsters are Effect Monsters with the "Gemini" ability. They are treated as Normal Monsters on the Field and in the GY, and they can gain their Effects if you Normal Summon them a second time.
While a Gemini Monster is face-up on the Field or in the GY it is treated as Normal Monster. It is treated as an Effect Monster everywhere else. You can Normal Summon it again to "unlock" its Effect(s).
Most Recent Gemini Card Text
This Card is treated as a Normal Monster while face-up on the field or in the GY. While this Card is a Normal Monster on the field, you can Normal Summon it to have it become an Effect Monster with this Effect.
During the Summon
The Gemini Monster is not treated as on the Field during the Summon. However, any Equip Cards, Counters, and modifications applied to the Monster are not lost.
Level 5 or higher
Tributes are not required for Level 5 or higher Gemini Monsters to be Normal Summoned a second time.