You need to satisfy the requirements of the powers you use, the rules are explicit about that especially in terms of legal targets, and nothing in PG changes the OA power. Nor should it, considering it pre-dates OA-as-a-power.
How does Polearm Gamble NOT alter Opportunity Attack; it names it
by name.
The fact they've changed how exactly the game element of that name works does not suddenly render every single thing that mentioned opportunity attacks previous to essentials null and void! By logical extension, you've rendered Combat Challenge useless! Heavy Blade Opportunity is no more! Artful Dodger suddenly does not work! If it says 'Opportunity Attack' but was printed before Essentials, it's refering to some game element that no longer exists rather than the game element that currently exists with that
exact same name?!?!
I'm just not touching that argument. It is
rediculous. Polearm Gamble refers to opportunity attack by name. It does not matter that it's a power now, what matters is that Polearm changes how it works just as it's always changed how it works. The exact rules mechanics of those changes might change subtly, but being a power does not render Opportunity Attack immune to specific exception.
But it doesn't say so. It says 'Make an OA against the target'. OK, I use that power... out of range, fails, done. For the exact same reason that a Polearm without Reach would fail, the power lacks sufficient range (MBA, in the latter case).
If that weren't true you'd never need the text "regardless of range" that exists in dozens of feats, items, powers, etc., that also grant attacks with odd triggers.
By dint of you SAYING this, you've identified a contradiction in how a specific rule interacts with a general rule. 'The rules break' is not the default option here. The default option is 'The specific rule takes precidence.' You don't stop halfway through the process, you resolve any contradictions in this manner, with the specific case overriding the general case
so long as it remains specific.
Polearm Gamble is specific to Opportunity Attack, so it can except it in any way it damn well pleases. Polearm Gamble is not specific to Melee Basic Attack, so the only rule that CAN apply there is Melee Basic Attack, with one single exception, and that being that Opportunity Attack allows the use of MBA without spending a standard action; this point is not addressed by Polearm Gamble, so is irrelevant to SvG.
So, of course you can't use a non-reach weapon, because there is no exception to Melee Basic Attack in play. Therefore, Melee Basic Attack takes precidence. However, you can use a reach weapon, because Polearm Gamble changes how Opportunity Attack works, because it's explicitly doing so, and will in every way it has to to MAKE IT WORK.
'It does not work' isn't an out for Opportunity Attack vs Polearm Gamble. It IS an out for Melee Basic Attack. Do you not understand the difference?