Hi. Was the upgrade an official Samsung one. If so then the issues you are having may be resolved when you do a factory reset. Before doing that though, you can try clearing the cache (done by putting the phone in Recovery Mode - powering on by pressing the Volume Up and the Home button. Look for an entry that says something about wiping your cache)
Then if this alone doesnt work, then you will have to do a factory reset then wipe the cache again.
If this works out, you shouldnt need to downgrade.
If you insist on a downgrade however, you will have to find a the official I9100 Gingerbread ROM from the internet on a site called sammobile.com.
You will have to flash it manually though once you have downloaded it. Do some research and always follow flashing instructions or you WILL brick your phone.