IMO it's absolutely no different to which club you supported as a kid. As a kid you didn't give a shit about money, lifestyle, coach etc, you just loved your team and were thrilled if they won and devastated if they lost.
That continues as you grow up, though generally the emotion lessens a bit (depending on the game, obviously grand finals etc are something different).
But seriously, a player who grows up, ends up being signed by another club and plays against the club they loved as a kid in a grand final...do you think it causes conflict in them? Maybe. But they still do the job and would still be elated when they win and devastated if they lost.
Absolutely no different to Origin. Gillett might well remember his supporting the Blues as a kid, but when playing for QLD, he wouldn't give a shit about who he used to support. He'd be all about winning for his team.