first go to your weapons directory in game folder, check for a file that resembles the grav gun...unpack it - and replace the modified grav gun file with the simple grav gun file...

now run the game, if you cant find the changes...use impulse 101 cheat.

Well this is just a hint. if you want to see a video tutorial, go to youtube and search for portal gun in half life 2...

in there it shows how to add a portal gun from the game portal (in the orange box with half life 2) to half life 2..

If you don't own orange box, just refer to the video and learn how to add the super grav gun instead of the portal gun.

hope i have helped somewhat...