I have a GGS1 and it is a very nice pack - solid, sturdy, padded back, shoulder harness and waist belt.  Comfy.  I'll carry up to 3 people's worth of personal gear in it.  As JS said, big enough to comfortably carry what you feel you can manage. 

The GG traditional #4 is super for even larger loads, but has no padding on the back - easily solved with a closed cell pad.  This also makes the #4 much more compressible.  The back on the GGS1 does not fold, so you have about a 18 in. square to deal with.  If you travel with the blue 30L barrels, two will fit in it side by side, but not so in the GGS1 - though they will stack sideways - inconvenient. 

For heavy 'crew' gear or community stuff - cook kits, axe, fuel, etc., I have an old Kondos food pack that is the right size without getting too heavy. 

The GG#4 is $157 at REI and if you are a member and plan it right, use a 'member coupon' and put it on sale for 20% off.  You also can't beat their return policy.  They do carry some other ones too. Check with some of the Ely outfitters that turn over their gear regulary as well.  Piragis sells most of their gear every season and I got a GGS1 from them in great shape at a very good price. 

Any way you go, make sure you have the side grab handles - makes it much easier to load in/out of the canoe.

pd