I'd imagine the gambling version is just an offshoot of the normal slang definition. Specifically....'Mainstream'. (Subscribes to conventional wisdom and/or groupthink & bets accordingly.)
(1) Unexciting, unadventurous, mainstream or dull. Generally used in describing a person with such attributes or who plays it safe.
(2) Someone out of date or out of touch, can also be someone who clings to repressive, traditional, stereotypical, one-sided, or "in the box" ways of thinking.
Historically, derived from a person whose goal in life is to have three "squares" a day.