Hi, D Dash!
There are direct buses available from Gangotri to Yamunotri and from Yamunotri, you can take a bus to reach Kedarnath. Else you can travel in shared jeeps or take a direct taxi from Gangotri.
To reach Gangotri your need to trek down to Gaurikund from the Kedarnath Temple, then continue by road from Gaurikund to Guptkashi to Rudraprayag to Uttarakashi, from there take shared taxi/jeep to reach Gangotri.
The route from Kedarnath to Gangotri is: Kedarnath → Gaurikund → Guptkashi → Rudraprayag → Uttarkashi → Gangotri