You must use version VS 2022 17.9.0 Preview 2 or newer to edit Windows or Dynamics GP forms in the forms designer of Visual Studio.
Since Visual Studio 2022, it is now a 64-bit application, this means editing Windows Forms in the forms designer for forms of 32-bit applications (such as Dynamics GP), is not possible (unless on this version of VS). Visual Studio throws an error: "The designer could not be shown for this file because none of the classes within it can be designed"
Note: Version 17.10.0 Preview 1.0 seems to have broken the designer again, this is the danger of previews...
Visual Studio being 64-bit will not host the 32-bit application form. This is not just a GP issue, anyone working with 32-bit components faces the same issue. This renders the forms designer in Visual Studio useless for 32-bit applications or visual components.
In this very long thread on the Microsoft forum, you can see the long wait for a couple of years for the solution to the problem.
The Visual Studio team needed to create a 32-bit process container that the form could be off-loaded to for editing within Visual Studio. The solution is not perfect for everyone's needs, particularly with complex components, but it solves the issue for Dynamics GP development and Visual Studio 2022.
Simply download the preview version of Visual Studio (be aware this is a preview version and subject to instability), or if you are reading this in the future, just download the current version as long as it is at or above 17.9.0, then you are good to go. Preview 1 also had the out-of-process forms designer but it was an option that had to be turned on in settings.
The out of process forms designer experience
When opening the 32-bit form, the dialog "Visual Studio is unable to load 32-bit assembly. Do you want to use Windows Forms out-of-process designer?" will pop up. There is also a difficult to read check box to make this selection sticky for this project (Remember for the current project). I have overlayed my label on the screenshot to make it easier to read.
The standard loading message then pops up. It took about a minute and a half for the form to actually open on the test project I was using, so make tea.
Then once open the top of the designer window remind you that this is an out-of-process designer with this message info on the top of the designer window.
The designer "feels" a bit like an extension to Visual Studio when in use, but it gets the job done in the limited testing I just tried for ten minutes before writing this up.