Understand the two constructs that define Dynamics GP — the shared segmented account framework and the distributed posting setup — and why sharing a vendor with D365 F&O makes this migration harder to scope, not easier.
Every migration carries a hidden prerequisite: understanding what the source system actually is, rather than what it appears to be from the outside. With Dynamics GP that prerequisite is unusually easy to skip, because GP and Dynamics 365 Finance & Operations share a vendor.
A typical GP estate contains one system database and a set of company databases. That split determines how discovery has to be run.
GP's defining accounting primitive is the segmented account string. A framework decision made once — often more than a decade ago, often by someone who has left — fixed how many segments exist and how long each one is. Every company database inherits it.
GP's second defining construct is the way it decides which account a posting lands on. The mechanism is not one table; it is a hierarchy of setup and overrides. In broad terms, account selection can be influenced by:
GP's Sales Order Processing and Purchase Order Processing modules are not one-to-one copies of D365 sales and procurement flows. SOP can blend shipment and invoice timing differently per document path.
GP estates routinely look simpler than they are, because a great deal of behaviour lives in client and report layers rather than in anything a source-control repository would recognise.
This path is built on Microsoft's public documentation for GP plus a reconciled adapter package. It is worth being explicit about the boundary of that evidence, because a confident-sounding design built on invented specifics is worse than an honest gap.
Dynamics GP and D365 F&O both produce audit-defensible, period-closed accounting records. They reach that outcome through different abstraction choices, and the migration is the work of re-projecting one set of choices onto the other.