I recently started selling my software on Gumroad. For the Windows side, we just zipped up the .exe installer and uploaded it. For the Mac side, we first made a disk image where we laid out the files and then compressed them. You can do this with Disk Utility, but it’s a lot easier to do it on the command line.
The first thing you should do is put all the files you want to distribute into a folder. Then get info on the folder to see how much room you will need for your disk image. Round up a little. In the example below, I’m creating an image with 20MB. “ ProductName” is the name of the software, e.g. Match Ups!, or Train Time.
hdiutil create -megabytes 20 -fs HFS+ -volname ProductName ProductName.dmg
Once you have everything laid out how you want it, compress the image.
hdiutil convert -format UDZO ProductName.dmg -o ImageForDistribution.dmg
Where UDZO – UDIF zlib-compressed image. You can rename ImageForDistribution.dmg to anything you want.