I really should check my quotes before I post, rather than after I get home. Miss Gilbert was not there. In ch 29, we have the revelation :

"Somebody said that Miss Gilbert was expected at her brother's, and must be invited with the rest."

I was tossing and turning to an irresistible country measure all night as a consequence, tormented with vulgar visions of the Miss Coxs dancing - sometimes with the Messrs Cole, Weston and Gilbert, and sometimes with each other.
It has not cured me of suspecting Mrs Cole however - far from it. The dinner seems to have been put on to honour Frank Churchill, (I infer this as he is new to the town) with Emma as the principle lady. I would expect Mr Cole to have lead the set with Emma, and suspect the Coles are involved in a conspiracy to ensure Frank Churchill and Emma Woodhouse led the set.
I am not sure if there were any Miss Gilberts - the Gilberts I find seem to have only one son, grown up.
This means the party was very equally matched with ladies and gentlemen when it came to dancing, and as Mr William Cox could not be expected to have stood up with one of his sisters, it seems that Mr Gilbert has stood up with one of the ladies, with his wife sitting out.
Gallant Frank Churchill shows his sense of duty when he comments to Emma that he would have been obliged to dance with Jane Fairfax next - unless ornery Mr Knightley could be persuaded to stand up with her...but I am not sure his kindness would extend so far. After all, he did not dance the first set, and, to our certain knowledge, Mrs Gilbert was sitting down in want of a partner.