When the title screen appears, press fire and you will go to the
options screen.  Select "ABOUT GLOOM" from the list of options and
press the fire button.  Now hold down the 'HELP' key and press fire on
the joystick.  The screen will flash purple to indicate the cheat is
active.

Now start the game.  The following keys are now available:

        '1'-'5' _____________ Select (and upgrade) all weapons
        '0' __________________________________ Increase health
        'HELP' ___________________________________ Skip levels

You can increase the strength of each weapon by pressing the number
key up to 5 times.

On disk 2 of gloom in the dir 'misc' there is a file named 'script'.
Use an XPK decruncher or Crunchmania to decompress this file.  Now
load the file into a text editor and you will be able to edit the
script to start at any level.  Now save the script and recrunch it
with Crunchmania or an XPK packer that supports Crunchmania.  I used
XFDcruncher/decruncher, which is available on Aminet.