Goto is really one on the most asked feature. (See http://plantuml.sourceforge.net/qa/?qa=tag/goto )
As said in http://plantuml.sourceforge.net/qa/?qa=1626/there-refer-existing-activity-inside-activitydiagram-goto it's very difficult to implement some goto feature with the current architecture, because the whole drawing is done with some recursive algorithm, and the goto completely breaks this.
Anyway, we have started to implement an alpha version of goto. It's been working for a while. You can try:
if (some test) then
Unfortunatly, the drawing is not very good, and it works only with goto going backwards.
Anyway, we are still very interesed with real world examples, so you can translate your old Fortran code to this syntax and send it to us.
It will help us to define a consistent architecture for supporting goto.