From Bulbapedia, the community-driven Pokémon encyclopedia.

A Gamer (Japanese: ギャンブラー Gambler) is a type of Pokémon Trainer that first debuted in the Generation I games. They are generally depicted as old men with dice and a dice cup. In Generation IV, they are young men in trenchcoats with coins.

In the past, they've been known as Gamblers, but this was changed due to ethical reasons to Gamer in FR/LG (occasionally causing slight problems, such as in FR/LG when a gamer describes himself as a "rambling, gaming dude" instead of a "rambling, gambling dude.") In Pokémon Diamond and Pearl they are called PIs, implying that they are detectives rather than gamblers, but their dialogue is all still based around gambling.

They use various Pokémon, but frequently employ moves with random effects such as Metronome.

In the TCG

There is a card called Gambler in the Fossil expansion of the TCG. It causes the user to shuffle their hand into their deck, then depending on the flip of a coin - they draw either 1 or 8 cards.

Appearance

Gamer Sprites

Image from Pokémon Red and Blue

Image from Pokémon FireRed and LeafGreen

Image from Pokémon Diamond and Pearl

Generation I-introduced trainer classes
Beauty | Biker | Bird Keeper | Black Belt | Boss | Bug Catcher | Burglar | Champion | Channeler | Cooltrainer Cue Ball | Elite Four | Engineer | Fisherman | Gambler | Gentleman | Hiker | Jr. Trainer♂ | Jr. Trainer♀ | Juggler Lass | Leader | PokéManiac | Trainer | Psychic | Rival | Rocker | Rocket | Sailor | Scientist Super Nerd | Swimmer | Tamer | Youngster
Beta classes
Chief | Pokémon Prof.