Gaster is one of two monsters that attacks with the help of Gaster Blasters, the other being Sans. But while Sans adds them to the owner's hand at the end of each turn, Gaster adds them to the deck at the end of each turn.
The Gaster Blaster deals 8 damage to a target of your choice. But if Gaster isn't on the board, the Gaster Blaster will only do 5 damage to a player. Like any other spell card, it can bypass Taunt. The Gaster Blaster costs 7G to use, so Integrity is a good soul type to use Gaster with. Alternatively, you can use Gaster's high health to your advantage by using Protection (in Patience) or Glad Dummy (in general) and giving it Taunt. However, it is still susceptible to easily dying via Muffet's Pet, or being weakened by having its stats swapped around with Madjick, so have a Snowdrake or other such card on standby to give it an increased attack stat. Useful after using Snow Storm on a full enemy board.
Since it states that all Gaster Blasters do 8 damage, it may even help the other player.
As a side note: Just like any other card giving cards, a Shiny Gaster will give Shiny Blasters.