OA has a specific limitation, it can only target up to 1 square away. PG has no text that overrides this. The situations are directly analogous. You still need to follow the general rule without an exception. OA's only working at 1 square away is the general rule, you've got diddly for a specific rule to change it. Literally could not be more comparable.
Except that Polearm Gamble specifically changes how Opportunity Attack works. You can't claim that it changes the trigger but that it cannot then work because it 'didn't change the range.' Cause, by that same logic, it couldn't then change the trigger either. Polearm Gamble doesn't 'explicitly' change only the trigger... it explicitly allows you to do something Opportunity Attack does not. It's very specific about what it allows you to do. It doesn't have to get Opportunity Attack's permission to change Opportunity Attack beyond saying 'You can now use an Opportunity Attack in a way you could not before.' That's all it needs to say. Because now you can. You don't get to pick and choose which subset of the thing it called out affects it. It calls out Opportunity Attack, Opportunity Attack is now its bitch. It is specific in doing so, it doesn't call out 'powers' or 'game element'. It says Opportunity Attack by name. It can't NOT change OA at that point.
Power allows a Stealth Check.
Feat allows an OA, which is a power (with no other provisos).
Power doesn't contradict the stealth rules. Stealth rules still apply.
Feat contradicts OA. OA's status as a power is irrelevent, feat takes precedent.
You meet the requirements for Stealth? No? No check, sorry.
You meet the requirements for targeting that guy with an OA? No? Oh, sorry, no can do.
The difference is in contradiction. The power does not inherently contradict the rules for stealth. Whether you can or cannot stealth is actually irrelevant to whether you did so through movement or through a power. The rules for stealth apply to both, and the rules for stealth are clear that they can apply to both situations. That makes stealth more specific than a power that simply allows you to make a stealth check. Moreover, these powers are
not changing how stealth works in their situations. There's no specific situation here, just a general one.
The feat creates a condition where, as you've pointed out, it is impossible to use the feat because 'the rules for Opportunity Attack disallow it.' In this case tho, the feat is clearly changing how Opportunity Attack works. You've even admitted it, it changes the trigger. Because it is changing how Opportunity Attack works, you clearly DO have a specific vs general situation. And, as you've noted, Polearm Gamble doesn't work unless it overrides Opportunity Attack, then the rules have a provision which cause it to do so. "Polearm Gamble wins." Once you've actually acknowledged that Polearm Gamble is a specific change to Opportunity Attack, then you must therefore acknowledge Polearm Gamble wins. You don't have an out 'Polearm Gamble wins by not winning' or 'Polearm Gamble is the goggles and does nothing.' Once you have a specific contradicting the general, you make it work, because
the specific must win.
'Win.' That's the exact word used. Not 'alters only what it says it can verbatim even if the element will not work.'
It wins. And if, in your interpretation, Polearm Gamble cannot win over something it explicitly mentions in its own text,
then you are wrong.
Give it a couple of months. It'll be errata'd with some very simple explanation like "by a strict reading of the rules the OA power having a range of 1 made certain game elements non-functional." Then you'll have a better understanding of what SvG actually is and what it isn't.
Except if those game elements explicitly call out Opportunity Attack, nothing in opportunity attack can inherently make those elements nonfunctional. That's just not how 'The specific wins' works. It doesn't matter how range works, if it says Range 1, and that won't let Polearm Gamble work, then by specific vs general, guess what the rules say wins?
Here's a hint. The specific. And nothing in Opportunity Attack is specific to Polearm Gamble. Everything in Polearm Gamble is specific to Opportunity Attack.
By a strict reading of the rules, Polearm Gamble wins. Every other rule is a slave to that first one. EVERY rule is a slave to that rule.