I have not printed a disk cover in several years. The time involved, disk vibration, poor adhesion, dual layer media, external drives, Sharpie pens and especially the expense of the paper media / cover disk media and burn drive was the reason I stopped making disk covers. The size mentioned below may not be correct for your disk media or . . . in the case of the burned disk image . . . correct for the software used in printing / burning the disk images. Since the image can be re-sized it should work in most cases.
To make a template for disk images you would need to measure your disk paper media to determine the size of the image required. I measured an old Fellows (Neato) disk sheet and found the disk size to be 4.625 inches with a 1.5 inch center hole so . . . I would create a template that is slightly larger to insure the media was covered entirely when printed. I would make a new image with a white background at 300 ppi and 4.750 x 4.750 in size. You don't really need a background layer but I like having a background layer as it makes it easy to change bg colors on the fly.
I would then add a transparent layer. Using guidelines, place a 5 pixel 4.625 inch OD circle and a 5 pixel 1.5 inch ID circle on the transparent layer. I would do this in order to turn the disk size on and off as I worked on the disk image. The disk image would reach from edge to edge with the disk size guides used to make sure the image would cover the media completely from outer edge to the edge of the inner cutout and that titles would be "on the disk". I would not cut the inner circle out of the image but use it simply to determine where the center hole would be during the design process. The template would look like this (attached image a).
Go find an image of the original disk or start making your own. Since I like to make game covers like the original I found the images for Planescape Torment on MobyGames. Since PS:T had four disks I decided to use the disk one image which would bring you to the question "Do I want to make a image for each disk so folks will have a choice of which one to use or . . . make just one disk image or . . . create my own disk image and disregard the originals ?".
Pop the disk image into your template between the disk outline and background layers and size to fit. Because the disk image is dark, I locked the transparency on my disk outline layer and filled it with white to better see where the image should fit. Once placed, edit the image and save remembering to turn off your disk size layer. It would now be ready to print / burn.
As you can see (attached image b) it looks really bad. Since the original disk images can be found on-line and most folks store their games several to a disk or on hard drives I'm not convinced it is worth the effort to make the original disk images . . . especially with the poor quality images available. You can make your own but that can be time consuming as well since you would need find / create suitable images and resize the titles or create new titles. A big hole in the nameless one's chest with a need to resize the name of the game would require even more work after the basic layout (attached image c).
I applaud your enthusiasm and in no way do I mean to discourage you but . . . I simply don't have the time to participate in this project full time. Some questions I would ask myself. Are you going to make a disk image for every GOG game or just the ones you like? Are you going to maintain and update the thread where the disk images can be downloaded? Do you have the time to spare as this would be a huge project spread over many months / years? Will you continue to create and maintain the disk images even if nobody helps? I will attach the template blank which can be used to create your own.
The time I have available for covers is near zero now but I do wish you great success with this project. Hope this was helpful . . . =)
Maybe a stickie if the project is still afloat this time next year . . . ; D