Glad you signed up to Android Forums.
Unfortunately, you wont be able to downgrade to Gingerbread without rooting your phone. For more on the generals of that see: General Android phone FAQ (updated 9th October 2012)
. Rooting isn't for everyone, so if you decide to do it, read up on it beforehand.
If you tell us which S2 you have, I'll be happy to direct you to it's section, where there's a lot more about rooting that specific model.
Hope that helps, and that you enjoy your time here at AF!