Are there existing systems in place that your Dynamics GP system will need to integrate with? A good example of this in many companies is a sophisticated web application already in place for customer orders and billing. Other examples may be systems for employee time tracking, fixed assets, or shipping software.
Note
Excel spreadsheets, no matter how complex, are not usually considered existing systems. Existing systems are most likely to be other database applications or separately purchased software packages to accomplish specific tasks.
If there are existing systems in place, part of the implementation planning will be to decide whether to keep these systems and integrate them with Dynamics GP, or replace the functionality with Dynamics GP. Other approaches may be hybrids of these: keep some existing systems but replace others, or keep existing systems for now and replace their functionality with Dynamics GP in a more phased approach, one at a time, after...