Xgettext utility extract not only strings from GetString() calls. All settings of "Text" properties will be taken in account and added into PO template file.
label1.Text = "My string";
The GNU.Gettext.WinForms.dll contains "Localizer" class that can be used to translate all text properties of the controls on the form including sub-controls. Localizer can also revert translated text values to their original state.
// Place it in form constructor or in OnLoad() event handler
GNU.Gettext.GettextResourceManager catalog = new GettextResourceManager("Examples.HelloForms.Messages");
GNU.Gettext.WinForms.Localizer.Localize(this, catalog);
WinForms application under Linux