While performing Ghusl under shower, I do niyyat and I start the ghusl by saying Allah ho Akbar. When I am done with washing upper body till knees, i stop the shower flow and open the tap flow and put my legs under flow of water for ghusl. My question is that , does my ghusl becomes invalid beacuse there is little time gap in flow of water over my body when I switch from shower to tap?

Your ghusl is valid and in order