If it helps any I am on the same path. Search this forum and you will find some amazing replicas that other members have made for themselves. This should give you an Idea of where to start. I asked the same things as you did. There is no pattern per say but the patterns from Project puppet are always a good place to start. They would have to be highly modified to work for Gonzo. The main hurdle with Gonzo is finding and sculpting the reticulated foam to make his upper head piece with the nose and the lower jaw. Some have gotten really good results without having the Reticulated foam. It is really hard to find a source that is thick enough and affordable. One of the guys even cut out the mouth piece that connects the upper skull to the lower jaw to make it easier to create the hollow for your fingers. This is how I am doing mine. you will probably want to experiment on some cheap foam to learn how to carve the block down to the shape you want. The eyes are around 50 mm. Several people have said smaller might be better. The real trick here is to find a good front on and profile of Gonzo you like and blow it up on a copy machine until it looks like it would be proportioned to your hand and arm. You can then base all your measurements from that. I believe most have suggested Airbrushing the face and nose but I know some have done really well with floral paint.
As far as clothes, I found a pretty close match to Gonzo's Argyle vest and I found the same black fruit tie that he is wearing in the current movie on Ebay. You can hunt for 18 month baby clothes if you dont want to make them but the shirt will have to be an amalgam of several shirts as Gonzo's arms are so long but if you get a shirt where the arms are long enough you might find the collar too big. So taloring will need to be done. I expect I will build his head and body first then alter his clothing after.
Lastly, for his fur, many are using a navy sherpa faux fur. This looks good. i am also experimenting with getting regular dark blue fur and trimming it and then boiling it. There are some great tutorials about that as well.
Hope this helps. the research is part of the game and for me part of the fun. If you want to have the eye lids move, that is a whole added bit of study. Best of luck. DT