By default gmStudio authors .NET project files (eg. .csproj) to build for Platform=AnyCPU. You may easily alter this using an Author/Fix in your translation script:
Most of the default structure and other "boiler plate" content in .NET project files is defined by gmSL logic in the system configuration file, AuthorText.gmsl. An alternative to the above fix is to use a project-specific version of AuthorText.gmsl. The instructions for doing this are discussed in this article on Custom VB6 Language Replacement.