Your GK stays in the goal.....

For example: when the opponent is kicking the ball long forwards, and his striker gives a well timed pass, why did my STRIKER blocks the ball insted of my defender?

I guess that T11 sometimes hussled the players names during matches, but don't think that your GK will play as a fieldplayer.......when he keeping the balls out of the goal, there's nothing to worry about.

And you can't train him that way because he is a GK :P
Even so you can't train a striker to be a GK!

The GK positions is a very specialized position