I've been through a lot of GP integrations and most of them were very straight forward. In my current environment we have integrations from different sources including Salesforce to SOP module, IT Asset management to Fixed Assets module(including bar-code implementation), ADP to GL, and others.
The current work will be considered the most challenging as we're moving the AR subledger to a new Oracle-based billing software and GP will be processing General Ledger entries from this system. So what's involved? The following transactions will be created in the new system and all the journal entries will be push to GP - customer payments (Lockbox, Wired,etc.), invoices, credits, write-offs, refunds(AP entry) and others.
I guess the experts are telling themselves this is NOT much of a challenge, well not really and why? That's just the first part of this integration and since we're a subscription-based environment like Salesforce, we need to create deferred schedules for each document integrated. I'll stop there for now and let you create your own architecture for this integration.