Does anyone have any information on the gestational period of female guppies? I thought it was around 20-25 days, but it has been about 4 weeks since I noticed my guppy looked pregnant and still no babies. Any ideas?

It's been my experience that it can take anywhere between 20 and 45 days.
Good luck.

Don't forget to get a breeding trap or you will have no baby guppies!

Great advice above about the breeding trap. It actually differs than a breeding net.
Livebarer fish, like your guppy eat their babies as soon as their born if they get a chance to.
A trap allows the babies to fall down into a safe area away from the mother.
A few will survive inside an aquarium if there are a lot of great hiding places.
They tend to get sucked up by filters too so a net or some other device to block the filter intake valve can be useful as well.

The mother can go back inside the tank as soon as she has the babies.
The babies will need to stay in the breeding container or put in the breeding net until they are big enough to not fit in the other fish mouths.

Good luck

:) Thought you guys would like to know - we had babies last night. Late last night I noticed that the female looked different - like she was about to explode and she kept circling the plants and a big rock cave I have in the tank. I was going to stop after work today and pick up a breeding tank - but too late. I managed to find 4 of the babies and put them in a breeding net before I went to work. Hopefully I'll find more when I get home tonight. Thanks for the advice!

That's great news! Good Luck with them!