I confess to knowing nothing about Creating an EFT file format that follows CPA Standard 005. Fortunately Sheldon does and he explains the easy way in a new post.
Dynamics GP: Creating an EFT file format that follows CPA Standard 005
Payables & ReceivablesAccounting & Career
Archived post from 2014. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.