Microsoft Dynamics GP, current version 10.0 as of June 2008 is pretty flexible platform if you are looking for modifications, custom modules programming, add-ons and extensions.
As many mid-size ERP GP has multiple customizing tools and you have to get some excurse into each one of them to select the best in your situation. Before rushing into modifications overview, please think through the fact, that GP is owned and supported by Microsoft Dynamics subdivision (former Microsoft Business Solutions) and as such you should expect it to be tightly linked with such Microsoft technologies as .Net, MS SQL Server, MS Office, Sharepoint, SRS (or SQL Server Reporting Services). Plus, you should know the history of GP design ? it was originally designed by Great Plains Software in earlier 1990th with popular shell IDE and programming language approach: Great Plains Dexterity (compare to SAP ABAP or Navision C/Side ? shells were believed to resolve computer platform dependence). Now is the time to review: