There's no need to take gymnastics; everything you need to learn Aikido is contained in Aikido. However, there's certainly no harm in it, practicing gymnastic tumbling may well help your ukemi a bit, as well as providing good CV exercise, which is always a good thing.
You don't need to be in great shape though, the key to Aikido is accurate performance of the techniques, as I'm learning every time I wind up on my back! (hee hee!)
Here's one thing folks might think about
: My Mom came one day to watch a session, and she remarked on the dance-like quality of Aikido (she's a former stage performer). I'm wondering if taking dance would help one's Aikido? Hmmm.