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Players of Red Dead Redemption 2: Online (RDO from now on) have been requesting that Poker or other gambling activities be added to the game.

These were expected to be included, as they are present in the Single Player, and in the predecessor Red Dead Redemption.

Many players of the game are suggesting that Poker or other gambling activities can not be included because of legal restrictions on Online Gambling.

Notably, RDO features two forms of currency. In-game "Cash", and "Gold". While Gold can be purchased with real money (Micro-transactions), the In-Game "Cash" has zero interaction with Gold, and can not be purchased with real money. “Cash” can also not be redeemed for real-world money, prizes, or anything else.

Transferring of currency is currently not something available in the game.

“Cash” is earned in all activities of the game, and no prizes would be tied specifically to the game of poker.

If Poker was to be included, and restricted to only In-Game "Cash", would this be an issue regarding Online Gambling laws in a significant number of countries?

Of note:

RDO is an M-Rated (18+) game.

With conservative Pot Limits gambling would be a vastly slower way to earn in-game “Cash” compared to other activities in-game.

If Poker was to be included, and restricted to only In-Game "Cash", would this be an issue regarding Online Gambling laws in a significant number of countries?

It is hard to give a comprehensive answer because laws vary significantly among countries. I will focus on U.S. law, but other countries very likely adopt a more stringent definition of gambling (Muslim nations are at one extreme of the spectrum, since their religion prohibits games of chance).

Under U.S. law (or at least in some of its jurisdictions) the scheme you outline would not meet the legal definition of gambling. The Black's Law Dictionary defines gamble as "[t]o play, or game [...] involv[ing],not only chance, but a hope of gaining something beyond the amount played".

For a more formalized characterization of gambling, see Com. v. Irwin, 535 Pa. 524, 527 (1993):

The three elements of gambling are (1) consideration; (2) a result
determined by chance rather than skill; and (3) reward

There, the court distinguishes between reward and entertainment in that in the latter "the player can never "win" anything other than a prize worth less than the amount he has played", Id. at 529.

This is consistent with the case law from other jurisdictions. See Farina v. Kelly, 147 Conn. 444, 449 (1960):

[A] lottery is characterized by three constituent elements, namely, a
prize, a chance, and a price. [...] [A]s commonly understood, gambling
involves not only chance but a hope of gaining something beyond the
amount played

Lastly, since

In-Game "Cash" has zero interaction with Gold, and can not be
purchased with real money

it is evident that the purpose of adding Poker the game in the way you describe is pure entertainment, not for a player's expectation of reward as outlined in the aforementioned cases.