Microsoft Dynamics GP ERP belongs to Microsoft Dynamics family of applications together with Axapta, Navision, Solomon, MS CRM. Great Plains is open for modification and custom programming and you as software programmer, who has reasonable background in accounting and MRP business processes and logic should understand your options in customizing GP
Do not expect GP modifying process to be very easy, and it also may lead you to the option of subcontracting the programming phase to professional GP Dexterity developer or technical consultant