The version being sold by TGC will only work with PureBasic and will be discontinued with the release of PureGDK 2.0. See here:
http://forum.thegamecreators.com/?m=forum_view&t=177817&b=38p
PureGDK is not a plugin. It's a complete port of the DarkBasic Professional engine to other programming languages. You use whatever supported language you want and the DarkBasic Professional commands are added on as if it were an extension to that language.
Try PureBasic first and decide if you like the language and the forms designer. You can do that without PureGDK. PureBasic is a completely separate product and is not bundled with it.
Yes, most plugins are supported out of the box. The enhancement pack you mentioned hasn't been added but it will be in a later beta.
See here for a summary of features:
http://forum.thegamecreators.com/?m=forum_view&t=187939&b=38