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:
...
<Author>
<Fix name="PostEdit">
<Replace name="Force Set Debug Configuration to PlatformTarget=x86" lang="csproj">
<OldBlock><![CDATA[
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
]]></OldBlock>
<NewBlock><![CDATA[
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
<PlatformTarget>x86</PlatformTarget>
]]></NewBlock>
</Replace>
</Fix>
</Author>
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.