Run app that has expired by Apple fiat?
Your licence to use and run Space Gremlin are effectively bound to the purchasing Apple ID.

From this discussion, it appears you have chosen to abandon the Apple ID associated with this purchase:


  Since they abandoned help with the account, I responded in kind and abandoned it too.


By abandoning the Apple ID account, you have also abandoned any purchases and licences associated with the account. In this case, that includes being able to run Space Gremlin.

Be aware that you never owned or had an absolute right to be able to run the application; I do appreciate this is unlikely what you want to hear. The first paragraph of Apple's USA default Licensed Application End User Licence Agreement reads:


  The Mac App Store Products and App Store Products (collectively, “App Store Product(s)”) made available through the Mac App Store Service and App Store Service (collectively, “App Store Service(s)”) are licensed, not sold, to you.


From a developer's perspective, the Digital Rights Management (DRM) problems you have suffered are a compelling reason to offer non-Mac App Store editions of their software.

Older Version?

As a workaround, you may be able to find an older version of Space Gremlin that did not implement Mac App Store receipt checking – assuming a version once existed.

Alternatives

Have you considered using an alternative tool, such as the open source GrandPerspective?



Other tools and utilities are mentioned in the answers to these questions:


How can I figure out what's slowly eating my HD space?
What is consuming HDD space on my MacBook Pro?
How do I quickly find large files and folders on my mac?