A developer can build and develop GIMP using Windows. One reason to do so is to fix any GIMP bugs specific to the Windows platform. However, there is already a package of GIMP for Windows (at: GIMP Windows installers by Jernej Simončič.). A GIMP debugger or developer would still need to submit patches to the GIMP development team.
Building GIMP using MinGW
MinGW is an open source Linux environment for the Windows platform.
Partha discusses building GIMP for the Windows platform using MinGW.
Building GIMP using Microsoft tools
Building GIMP plug-ins using Microsoft tools
Packaging third-party GIMP plug-ins for Windows
Users on the Windows and Mac OSX platforms expect software to be packaged in a self-installing file (an installer). Even though GIMP plugins (whether scripts in Python or Scheme, or executables in the C language) are relatively easy to install, you might want to package them using an installer such as Inno Setup.