Grem is Australian slang for a novice or incompetent surfer.
Grenade Rifle Entry Munition
The form of gob meaning to spit something out. e.g. Did you see him grem? Yuck. Usually associated with that ghastly noise as the content of the lungs are coughed into the mouth before gremming can take place. Grem is also the word that describes the green lump that is created in the process. You might call it hacking up a hacker.