Would just like to say, excellent choice in Gargoyle's Quest. It's my favorite game of all time. I've seriously beaten it nearly 300 times. I still have my original copy and the manual. The box, and other Game Boy boxes I had as a kid, were thrown away by my parents, unfortunately.
I've been collecting complete in box GB stuff for a few years now and I have about 220, ATM. If this is a system you're seriously trying to collect, it will be slow going because decent shape complete in box stuff is pretty hard to come by for the system. There are definitely some grail-worthy rarity titles to be had on the system, and they are not cheap. Few examples:
Spud's Adventure: Had to trade my US Magical Chase to get it. This was the last one to appear on ebay, AFAIK.
The Flintstones: King Rock Treasure Island: The cart is fairly common but, it's estimated that there are less than 10 boxes in existence.
Amazing Tater: Had to trade King Rock to get it. Generally considered to be the rarest game on the system next to New International Version Bible.
For me, I don't intend to collect every game, as for every system, there's going to be a lot of crap games, unfortunately. Every game I want ATM is in the low to mid 3 figures. My most wanted right now would be Kid Dracula and Mr. Do!.
I would just like to clarify that I collect US versions, not PAL. The games I mentioned earlier are much more common in Europe. You can tell the difference by the Nintendo seals: US has oval seals and PAL has round seals.
Jon