We present a kinematical study of the optical ejecta of GK Per .
It is based on proper motions measurements of 282 knots from \sim 20 images spanning 25 years .
Doppler-shifts are also computed for 217 knots .
The combination of proper motions and radial velocities allows a unique 3-D view of the ejecta to be obtained .
The main results are : ( 1 ) the outflow is a thick shell in which knots expand with a significant range of velocities , mostly between 600 and 1000 km s ^ { -1 } ; ( 2 ) kinematical ages indicate that knots have suffered only a modest deceleration since their ejection a century ago ; ( 3 ) no evidence for anisotropy in the expansion rate is found ; ( 4 ) velocity vectors are generally aligned along the radial direction but a symmetric pattern of non-radial velocities is also observed at specific directions ; ( 5 ) the total H \alpha + [ N II ] flux has been linearly decreasing at a rate of 2.6 % per year in the last decade .
The Eastern nebular side is fading at a slower rate than the Western one .
Some of the knots displayed a rapid change of brightness during the 2004-2011 period .
Over a longer timescale , a progressive circularization and homogenization of the nebula is taking place ; ( 6 ) a kinematic distance of 400 \pm 30 pc is determined .
These results raise some problems to the previous interpretations of the evolution of GK Per .
In particular , the idea of a strong interaction of the outflow with the surrounding medium in the Southwest quadrant is not supported by our data .