When developing integrated applications using Dexterity, you will be working with a copy of the actual Dynamics GP dictionary. You will have access to all of the preexisting resources, except the scripts, and can re-use them in your application.
While it is possible to change existing Dynamics GP forms and reports and use them in your application as alternate forms and reports, best practices dictate that you should avoid creating alternate forms and reports because of the tremendous amount of work you might be saddling yourself with to maintain them. For example, with each new release of Dynamics GP, you quite possibly will need to recreate these resources from scratch.
In addition, you will be limiting your users because they can only use one alternate of the same form at a time. If two developers modify the same form, somebody's functionality will lose.