I've just tried the /ignore command via shortcut and it doesn't work:
path here\GameSaveManager\gs_mngr_3.exe /ignore Steam [Screenshots]

It sounds like there's many files that GSM is trying to process. Unfortunately, the 3x branch of GSM performs all actions within it's UI 'thread'. This essentially means that if the UI is interacted with (by the user or Windows itself 'pinging' it to check it's status), it can result in the application hanging.

As you mentioned, the commandline /ignore can be used to skip troublesome entries. However, you need to wrap it in quotes!
Example:

Yeah, after I changed the line like you suggested it worked.

Still, looking through the topics, I already see several other with exact same problem. Shouldn't this folder have been ignored by default? Usually people do not release programs with defaults that prevent it from working at all.

It's something that's been discussed (a 'default ignored' listing), but it's not really a great solution.
I'm hoping that the 4x branch--whenever it's finally matured enough for release--will address the root cause of this.