To get the blue gravity gun from the citadel to any point in the game bring up the console type in map d3_citadel_03 this is the exact point where you get the blue gravity gun. Then bring up the console again and type in changelevel (then the map name) e.g changelevel d1_trainstation_01 (taking you back to the start BUT DONT change the map as this will reset the weapons and you wont get the graivity gun). Then if the gravity gun isn`t available yet just type into the console give weapon_physcannon, and the blue gravity gun will be given to you (Just be careful when you aim it at vehicles that aren`t part of the terrain)