There's a long explanation in the manual at
http://manuals.ootpdevelopments.com/...acticsandroles - but in this specific case, the reason you can only choose those options for him on the second line is that the tactic you've chosen requires specific types of roles on each line. You may be able to get Gagner into a better role for him by clearing the roles for the other two forwards, since that'll give you three role types (each with several specific roles) to choose from - but that may mean there's no longer a good role selectable for one of the other two guys.