How would I change this sentence to active form?Guber made a spectacular play, but his shot that would have won him the game was blocked.

The beginning of the sentence is already in active voice, therefore, it is only the second part of the sentence that has to be changed.  The subject of your first clause (Guber) is performing the action (made) now, you have to make the same happen in the second clause.  In the second clause, the subject (shot) is not performing the action and in order for this sentence to be made into active, the subject of the second clause must be changed.  Your sentence written in active form would be:

Guber made a spectacular play, but a member of the other team blocked his shot that would have won the game.