T.,
How exciting! What a wonderful time to reveal the surprise to your mom. I don't have too many ideas, but here is what I can think of.
You could get her a bouquet of flowers in the appropriate colors
Maybe a charm for a necklace or bracelet for the sex of the baby
Maybe set of baby items for Grandma's House to include a special blanket (a replica of Baby's Blanket that s/he will use at home), bag, toy, etc in the appropriate colors
If she is a fan of candy, maybe a box of candies that are personalized for a baby boy or a baby girl - there are several companies that do this...m&m's, Hershey's, etc...
Maybe a special Christmas Ornament that will reveal the secret...like a train for a boy or ballet slippers for a girl...or something that will reflect your personality at the same time letting her in on the secret:)
If you are at all poetic, maybe you could write a poem that incorporates some of the things that are most special about your mom and how that will be able to help teach a little boy or little girl some wonderful life lessons
Maybe a special Christmas Stocking for Grandma's house in the appropriate colors...
You could fix a dinner for her and have foods "associated" with boys or girls...you would have to use the stereo typcial foods not for kids but think adults...men stereo-typically will prefer things like beer, brats, wings, etc...where women are more often portrayed wanting things like wine, chocolates, sweet treats, salads, etc... (trust me I myself cross the "gender lines" when it comes to preferred foods, but hopefully you get my idea)
Those are the limits of my creativity this morning. Have fun with the big announcement!
T.