First off, I think Amelia would playfully punch Gid when she got there. But she would have come dressed in this dress he left for her (pretend Sally helped, OKAY JULIA?? <3<3). At first she thought he was just elaborately leading her to some fancy dinner, but she really was surprised by the party.
At the beginning she would mingle and be cheerful and excited and thank everyone for gifts. Imagine lots of hugs and chat times and she might even make them dance even though they just had the ball.
Later on in the evening, she probably will have drank a lot of alcohol. Because it's her party! And she's still sort of internally meh-ing at the fact she's another year older. It's no ones fault really, I promise. But she probably would have gotten up and given an adorably hilarious but seriously heartfelt drunken speech about how thankful she was for everyone and how much she loved them and appreciated them. Because that is Amelia, okay? She would have especially thanked Sally too, because she knows she helped with everything and she's always there for her and like the sister she never had and yadda yadda. She probably went off on a few tangents before someone had to cut her off and shut her down.