Essentially if you travel to or from the UK, but not via a UK customs airport as your first/last airport of call. And if you travel via a UK customs airport, they (so I'm told) will most likely ask you to fill in a GAR form anyway.
The details as to whom to send it to, in what case, and with how many hours prior notice are on the back of the form. So are the fax numbers. But my limited experience has learned me that it's easier to call the airfield in question beforehand (needs to be done anyway for PPR) and fax the form to them. They'll fax it onwards to whoever needs to know.
Initially it may come across as very daunting. But the reality is that without the GAR form and the procedures that surround it, you would always have to fly to/from a customs airport, making your options quite limited. Right now, essentially, I can fly from any customs airport in Europe direct to any airport in the UK and vice versa.