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.
Air: You can head for Yamunotri in special helicopter services that start from Dehradun to Kharsali, which is about 6kmsaway from the temple. Rail: Nearest railway stations are Dehradun at 172kms and Rishikesh at around 200kms. Alternatively, Haridwar is also a good option. All three cities have excellent connectivity with major Indian towns and cities. Road: Yamunotri temple has no access to motor able roads; hence the base point is at Hanuman Chatti, at a distance of 14kms. For the adventurous kind, the trek uphill is scenic and challenging. Alternatively, there are ponies, horses and palanquins for the ride to the temple.