I deleted gas buddy by mistake, and the app store says that it is installed, how do I get it back?

If you go to Purchased under Store left hand side menu of iTunes on PC ( can't help with Mac,sorry )

at bottom right hand side should say "download Previous Purchases "

Then on next screen Apps and top right "not in my library" and you should see any apps you have already purchased (iTunes 10.5.2.11)but not currently in library

http://support.apple.com/kb/HT2519

You should be able to get it from your computer if you have synced your iPhone while you had it installed. If you don't want to do that, try restarting your iPhone and then check again on the App Store to see if it still says that it's installed.