Viswanathan Neelakantan explains How to pass parameters to custom SSRS reports activated to print from the Dynamics GP windows
How to pass parameters to custom SSRS reports activated to print from the Dynamics GP windows – Developing for Dynamics GP – Site Home – MSDN Blogs
Reporting & BITips & Hidden Features
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.