I was able to read your email, so you fixed it.
What led me to Gorm and opening the GNUMail nibs in it, was that in my debian 9 VM (that I use to build and run gnustep), I bumped into a problem. When I open the preferences dialog and in the accounts section I press either the Add or Edit buttons, they stay pressed and no new window appears.
So my first approach was to open the NIB file in Gorm and see which target/method was linked to the buttons.
Then I noticed that I could not select anything inside the panel. I wanted to select the button but no red selection dots appeared and the inspector didn't update to reflect the component I was selecting. Then I tried opening a gorm file and selection of components works for those files.
As to to GNUMail button problem, I am following the code in gdb, and right now I don't remember the spot where the error occurs and do not have access to the VM to get it. (I am at my day job). When I get home I can reply with the class and method where the I suppose the error is occurring.
As I understand, the GNUMail application is developed in Macs and then compiled for Linux, right ?