Hi there - seems that this is an issue with the VST version of this plugin. It locks up if it receives this host automation on a certain thread.
The AU version seems to work better though so please try to replace your plugin with the AU version and see if that helps. It would help the developers of the plugin if you reported this issue to them as well.
As a side note … I actually prefer running the MIDI Guitar 2 as a separate application and then using a MIDI In block in GP. This allows me to globally adjust some settings of MIDI Guitar 2 and offload some of its processing outside GP which helps with the overall performance.