Body Repair
Lets see if we can get ALL of the Canadian Opeler's in on this post.
As Gary mentioned, the flat spot in the inner fender just behind the wheel forms a lovely shelf for wet mud to collect. The result is the inner fender rusts, and often the outer fender as well (from the inside). On the passenger side, this is directly above the heater fan and core, which allows mud and stuff to rain in on top and can wreck the fan motor. Check this out before you repair the inner fender.
But I think you may actually be referring to the outer fender, just behind the front wheel, down near the bottom. This is also a favourite spot for mud to collect.
To repair either of these, make sure that you have removed ALL of the rusty metal, whether you mig weld steel (my prefered technique) or use some type of glue or even (!) pop rivets. But as a warning, if you use rivets, avoid the temptation to use aluminum rivets, and especially aluminum sheet. I learned the hard way on my first "restoration" (a '61 Austin Healey Bug Eye, back in 1975) that aluminum and steel forms a galvanic cell (a "battery", if you will) which will greatly accelerate corriosion. My second attempt used steel sheet, but I brazed the panels. Lasted much longer, but the brass is also "dissimilar" (different spontaneous potential, for you chemistry fans) and unless it is made very water proof (no water, no electrolyte, no cell, equals no corrosion) it will also rust prematurely.
Finally, to keep mud from packing back in these spots (and these are only two of many mud-collecting locations on a GT), I thoroughly coated the repaired area with zinc-rich primer (back before the fancy rust-preventing coatings) and then used rust paint, and finally rust "proofing" (really just tar), THEN I filled those areas with expanding foam to act as an inner fender. But you could also fabricate a true inner fender, or just be very diligent in washing those areas out once a week.
I posted a site in the "Body" section of a guy in Belgium
http://home.tiscalinet.be/rentfarm/Opel-GT/index_bestanden/page0005.htm
who did a very well documented body restoration, and you can see some of the troublesome areas, including the inner fender shelf.
HTH
p.s. I am going to move this thread to the "body" section, so it will be easier to find and that section won't be so lonely!