OP has not told us what type of flight, how he is supporting the UG (or whether it is a freebie), and how he knows how many seats are for sale.

But, for whatever reason, if there are any seats available and UA believes that it can either sell them or that it will need them to acomomodate paid F in IRROPS, why on earth would it do these UG's before the gate?

There is, in fact, a compelling argument not to do any freebies before the gate because if they can be sold that is better than handing them out.