I have bought your App on the App Store, how can I transfer it to my Mac/PC?

If you purchased the App directly from the App Store for your Apple device, you do not have a Desktop license for it.

iOS Apps and their Windows or Mac counterparts are two separate things, they require two separate purchases, even though the software is basically the same.

This is true for 99% of the software available for both Desktop and Mobile platforms. There are only a few exceptions of software that allow some sort of cross-grade licensing. Of the GSi software, only VB3-II allows you to transfer a Desktop license to the iPad, but all other plugins are separate purchases.

The same is true between iOS and Android: whenever the same App is available on both platforms, there is no exchange mechanism, they are separate purchases.

This is not only true for GSi, but also for all software from other developers. It is simply the way the mobile software paradigm has been implanted since day one.