I'm busy planning my first flight from mainland Europe to the UK, and the whole concept of the GAA is very daunting. Depending on where you fly from and where you fly to there are four different notification periods (3, 4, 12, 24 hours), three organizations (Customs, Immigration, Police) that may or may not need your details, a zillion fax numbers and there's another thread here somewhere suggesting that half of these fax numbers are outdated anyway.
For starters, why not include two simple tables instead of the masses of text there are now. One table about arriving in the UK, the other departing the UK. Horizontal axis: to/from a customs airfield, a GAA airfield, a non-GAA (private) field. Vertical axis: To/from the CTA, the EU and non-EU countries. And then in the table you list which organization needs to be involved and their notification period. (And in a few cases, such as non-GAA to non-EU, forbidden altogether.)
But my preference would be a single web site where I input my origin and destination, and it determines automatically who needs the info, and gives me the notice period required. Failing that, a pdf form which can be filled in on the computer and an e-mail address to send the filled-in pdf to. And failing that, a single fax number to fax the form to, and someone at the other end to sort it out.
My experience so far is that if you're flying in from mainland Europe to any field in the UK, your best bet is to fax the form to the field you're flying to. They'll send it to the proper authorities in time.
From a design point of view, the "Home Address" field is on the smallish side and why is the "via" important on departure but not on arrival. What is required in the "via" field anyway? An airport, a VOR or other waypoint? And why do you use three different fonts (and ugly ones at that too)? Oh, and on the back there's a big grey bar. Initially this looked like a divider between two sections. I only found out much later that it contained the most important fax number of them all.
Also what needs to be made clearer is what happens if your flight is delayed for some reason. Do I re-fax another form, is there any way I can cancel a GAR form once sent? Or are customs waiting for me needlessly, or do they have access to FPL data with my revised ETA?
Other than the rant above, I really do like the concept where you can fly into and out from the UK other than via a customs airfield, provided that certain criteria are met and a reasonable notification period is taken into account. And in an ideal world, the UK would subscribe to the Schengen agreement and all that would be required would be a flight plan sent 1 hour in advance.
Hope this helps. Good luck!