Gemini Monsters (known as Dual Monsters in the OCG) are a type of Effect Monster. They have effects, but these effects must be "unlocked" by performing an additional Normal Summon on them while they are Face-up on the field (known in the OCG as a Second Summon). Until then, they are treated as Normal Monsters.
All Gemini Monsters to date are treated as Normal Monsters while they are face-up on the field before being Second Summoned, or while they are in the Graveyard. If they exist anywhere else, they are treated as Effect Monsters.
The process of Second Summoning a Gemini Monster (that is being treated as a Normal Monster by its effect) is just the same as performing any other Normal Summon, except that the monster is already on the field and you don't have to Tribute monsters for Gemini Monsters of Level 5 or above. You cannot Second Summon a Gemini Monster that has already been Second Summoned (and is still considered to have been Second Summoned).
Once a Gemini Monster has been Second Summoned, it is then treated as an Effect Monster and it gains the effect(s) written on it.
Types of Monster Cards
Types of Spell Cards
Types of Trap Cards
Other Types of Card
ATK - DEF - Level - Rank - Attribute - Type
Playing Field Zones
Types of Summon
Types of Monster Effects
Types of Effect Monsters
Types of Spell Speed
Aqua - Beast - Beast-Warrior - Cyberse - Dinosaur - Dragon