This seems to be a work around but I believe it can help resolve your issue. Atlassian has a free plugin call Bamboo Inject Variables Plugin. Basically, with this plugin, you can create an "Inject Bamboo Variables from file" task to read a variable from a file.
So the idea here is to have your script set the variable to a specific file then kick off the build; the build itself will read that variable from the file and use it in the grails task.
UPDATE
After a search, I found that you can use REST API to change plan variables (NOT global). This would make your task simpler: just define a plan variable (in Plan Configuration -> tab Variables) then change it every time you need to. The information on how to change is available at Bamboo Knowledge Base