How can I open an older type Gustavsberg cistern toilet?
This page details two different sorts of top-flush toilet repairs:

http://homeguides.sfgate.com/fix-flusher-top-toilet-66536.html


  Unscrew and remove the knob from the end of the rod that goes through
  the top of the tank cover. Spray lubricant on the rubber grommet that
  seals the rod where it goes into the hole in the cover. 2
  
  Push down on the end of the rod as you lift up the cover. Take the
  cover off the tank. If necessary, lift up the cover enough to grip the
  rod at the underside of the cover, using vice grips. Lift up on the
  cover and pull the rod out of the grommet. Set the tank cover aside. 3
  
  Fit the vice grips onto the rod and rotate the rod a bit each way
  several times as you pull upward. In some cases, mineral deposits can
  build up and prevent the rod from pulling upward. If the toilet
  flushes, reattach the tank cover in the reverse order and screw on the
  knob. Alternatively, if the toilet doesn't flush, enlist a plumber to
  install a new flush valve and rod mechanism.


The quote above is for the "rod-activated" version.

You mentioned that the knob feels tough-- try using a penetrating oil on it, it may help. It appears as if the rod sits inside of that knob... It also seems to imply that you'd have to pull it for it to work (yours doesn't look like it would be the button type).