I would argue that the GAs weren't ignoring you -- they were working to get the standbys they knew about on the plane. This is arguably the most time-crunched process they have to go through, because they are pressured to leave on time.

Could they have gotten you on (even though they didn't know you were coming to the gate, and didn't know why you were standing there while they finished their duties to get the plane out on time)? - maybe.

Is that (or should that be) their priority in the moment? Absolutely not.