Can't open app in macOS Sierra
I'm afraid the garbage collection feature of Objective-C was deprecated back in 10.8 in favor of Automatic Reference Counting or ARC. Now with the release of Sierra, this feature appears to be officially obsolete. Most likely this means the feature was completely removed from the Objective-C runtime, so there is no way to continue to run old applications that depend on this feature.

The application will need to be updated to the new ARC system to work on the newest versions of OS X. Alternately I suppose you could run it in a VM with an older OS X version.