I am very much an SMG fan and so my opinion on this issue is subjective, but I don't think there is any question of her showing a lack of gratitude. I've never really understood this accusation, one that has been levelled at her many times. Exactly who is she showing a lack of gratitude towards?
We don't know what went on behind the scenes. We don't know anything about Gellar's relationship with her co-workers, etc. For myself, I don't really want to know. I'm sure that over the period of the seven seasons of the show there would have been arguments and disagreements and conflicts, and not just involving Gellar. After all, these are all just people like the rest of us.
Much has been made of the fact she didn't attend the wrap party. We all know the official reason. She flew out to Canada on the same day she finished filming on BtVS (which had over-run its production schedule) to join the film set of "˜Scooby Doo 2'. That seems to me to be a perfectly reasonable explanation and it's one that can be easily verified, unlike the silly rumours and speculation.
It would have been nice to hear a few audio commentaries but she didn't do them, so that's that. We don't actually know why she didn't do any but if we simply assume it was because she didn't want to, that is her own personal decision and I really don't see how she can be criticised for it. In any case, if Gellar had done commentaries something tells me there would have been complaints about what she did or didn't say in them. She just seems to be a very emotive subject for some reason.
Apart from all that, Alyson Hannigan, Anthony Stewart Head, Emma Caulfield, etc, didn't provide any commentaries for the DVDs. Does that mean they showed a lack of gratitude?
Gellar doesn't do fan conventions. It's well known that she doesn't like crowds, as has been mentioned here. It's also been widely speculated that she just doesn't like conventions. Once again, that is her perfectly acceptable personal choice/opinion. Seth Green, Emma Caulfield and Charisma Carpenter have mentioned in the past that they are not particularly fond of conventions, although they have all done them. Gellar has done things of this nature herself from time to time, but it's obviously not something she wants to make a habit of.
She has been working continuously for virtually the whole of her life and I'm sure she has plenty of things to keep her occupied in her limited spare time. If attending conventions is not something she wants to do, so be it. Some actors get something out of that kind of interaction with fans. Others don't. Let's face it, if someone like James Marsters (a regular and very popular convention attraction) had several films in various stages of production (in other words, a packed work schedule) he would be far less likely to attend these conventions. We do need to be realistic about it.
For reasons I never understood, Gellar got heavily criticised by a lot of BtVS fans for (apparently) not inviting the people she worked with to her wedding. What on earth is all that about? Why exactly should she have invited them? I get on very well with the people I work with but I didn't invite any of them to my wedding and they did not expect to be invited. Gellar's private life is her business and the people she chooses to be her close personal friends is not something that should be the subject of public debate/criticism.
When asked, Gellar always says how much she owes to BtVS, although clearly (and understandably) she wants to move on now and get on with her career. In my experience, she has never publicly had a bad thing to say about any of the people she worked with. Contrary to some of the more ludicrous Internet gossip, many of those she worked with have had nothing but nice things to say about her and she does still maintain contact with some members of the cast and crew (Michelle Trachtenberg and Seth Green being the obvious examples). She is clearly a very ambitious and determind person - and her life and career exists outside of (and beyond) BtVS.
Gellar worked extremely hard on BtVS. Nobody can deny that. She helped to make Buffy an iconic figure. I think it's reasonable to suppose the show would not have survived as long as it did without her involvement and I'm happy to celebrate that fact, just as I'm grateful to all the actors, writers and crew who helped to make it the greatest TV show I've ever seen.