In DnD 4e, I wield a Glaive with Heavy Blade Opportunity and Polearm Gamble.
If my defender ally slides an enemy adjacent to me on his turn, can I use an at-will attack power on the enemy?
Benefit: When a nonadjacent enemy enters a square adjacent to you, you can make an opportunity attack with a polearm against that enemy, but you grant combat advantage to that enemy until the end of the enemy’s turn.
It does not, like other powers, indicate "on their turn." Furthermore, the natural impossibility of taking opportunity actions on your own turn prevents this power from triggering from your own forced movement.
Furthermore, the enemy must be non-adjacent on the move (defined as leaving 1 square for another, not the move total), so if the enemy starts adjacent to you, a slide of 2 squares is necessary for Polearm gamble to trigger.
The rules say no OA's are provoked by forced movement, and this was meant to include PG and/or it would be an unintended side effect to allow this to function like this.
Polearm Gamble could be expanded into a power card that would look like this:
Trigger: A nonadjacent enemy enters a square adjacent to you
Effect: You take an opportunity attack against that enemy and grant that enemy combat advantage until the end of its turn.
Rules Compendium p.200 states:
enter a square: Move into a square on the battle grid by any means, whether willingly or unwillingly.
Rules Compendium p.212 states:
No Opportunity Actions Triggered: When a target is pulled, pushed, or slid, it does not trigger opportunity actions, such as opportunity attacks, that are triggered by the movement.
In this case, Polearm Gamble's trigger condition is based on the keyword "enter", which is defined as a type of movement. This means that the No OA rule applies, because the Polearm Gamble OA is triggered by movement.
With that in mind, Pureferret does make a valid point that players have come up with an interesting teamwork strategy; the DM should certainly consider allowing the opportunity attack as a house rule if it makes the game more fun for everyone involved.
As Brian states, this is not 100% cut and dried. However I believe we can reason our way to the likely true answer.
The Rules Compendium, page 29 gives us Two Principles to Keep in Mind and number 1 is Specific Beats General. Therefore it would be easy to stop there and decide that Polearm Gamble is clearly allowing an exception to the general rule of No OA's during forced movement, but I don't believe that is the case.
Polearm Gamble is clearly allowing an exception, but what exception is that?
If the wizards team had wanted to make Polearm Gamble a strong feat they could have just said it granted Threatening Reach 2 (or perhaps +1 your normal reach). But they don't, clearly they wanted to limit it to something less. You aren't allowed
Looking at all of that, I find it hard to believe that it was meant to allow you to take OA's against shifting and creatures undergoing forced movement. Any other power that allows OA's against shifting creatures specifically says so.
Polearm Gamble reads like a power that was meant to be less than threatening reach, not more.
D&D is based on broad rules, with many exceptions. Polearm Gamble uses deliberate arduous wording depicting that any enemy not adjacent who enters, not moves, into a square adjacent draws an OA.
DMs can houserule it not to if they wish. But as written, OAs are granted.
As Brian states, it's in the rules for Polearm gamble, and forced movement OA's are only forbidden for movement away from the attacker. In this case the specific ruling outweighs the general.
The more important thing however is that two PCs have come up with a strategy that works using teamwork, which is something that should always be encouraged. If it proves too powerful it shouldn't be nerfed by the rules, but by the enemy wising up and being more tactical themselves.