This is actually quite a popular transition used in many songs, for example in Pink Floyd’s ‘Comfortably Numb’ song. Again you may try to isolate this part and practice for a few days going from G to G/Gb to E minor.
The chorus part features a quite unusual chord G/Gb (G slash G flat) that sounds pretty strange and tense on its own, but perfect when placed in between G and E minor – please watch the video lesson for a full explanation.