Cheat:
 There are two cheat codes for G-Sector (they were 
 really put in for testing purposes).
 Push F1 and F2 together. That will set the internal 
 level counter to the previous level.
 Then push ESC to bring down the menu and choose to 
 Continue. It will load the last level (unless you are 
 on the first level.)
 
 Push F3 and F4 together to go up a level.