I am wanting to make a little gumpaste girl (my first attempt at modelling with gumpaste) that will sit on my daughter's birthday cake. I am wondering if I could make this well in advance (?weeks) of the cake. I am a bit concerned that if the figure is dry and not pliable when I put her on the cake it will be difficult to situate her properly. If I am able to make her ahead of time, can I sit her on something along the lines of an inverted cake pan to dry?

You can make the gumpaste figures months in advnace but make sure it dries well before storing it
If you think it wont sit on the cake just insert a toothpick (where its supposed to sit) & let it dry as is. IMO its best to put the figure on a cooling rack for it to dry.

You can make them several weeks ahead with no problems. I would position the girl on a cake pan how you would want her to be on the cake to dry, because once she is dry you won't be able to move parts of her.