If I had to guess, I would say yes, Gingerbread will most likely be the last update. Samsung and AT&T just want us to upgrade our phones anyway to put more money in their pockets. Gingerbread will probably be the best thing that comes to the Captivate.
Samsung has officially said that they will not be porting ice cream sandwich to any galaxy s phones (which the captivate is). I'd be extremely surprised if att updated the phone without a build from Samsung, so it's a pretty unlikely to be updated any more. I was frankly surprised when gingerbread became available for the phone via formal channels.
Whether there is another gingerbread update is a good question. I'd again speculate that it won't be updated again.