Dear Dream 123,
as Rana sb has posted there are already a number of threads on Gorakh here to guide you,
Firstly you will not be able to go by bus all the way to hill summit as you will need a more nimble 4x4 and a driver with experience, I have been there often and seen several people fry their clutch because they don't know how to drive in offroad terrain, the gradients of the road are very steep, in some places 30 to 35 degrees so you really need to know what you're doing both going up and coming down,
Travelling from Karachi I would recommend the following route which I find to be the quickest:
Karachi to Jamshoro (M9)
From Jamshoro you need to take the N55 (Indus Highway) from where you can go to Dadu. It would be shorter if instead you take the N55 to Bhan and from there you can take a detour which takes you directly to Johi.
From Johi you will travel to Wahi Pandi and this is where you will need a 4X4.
Do not attempt to go further than Wahi Pandi on your bus as you won't get very far, there are a number of bridges from Wahi Pandi travelling to the hill summit and after last years floods the approach and departure ramps of these bridges were washed away so you need to offroad from the very first bridge.
I would recommend renting a 4X4 at Wahi of you do not already have one, the 4X4s available here are locally moded Potohars, they are only available with driver and each will cost you around 3500. You will have to fill up the diesel too, and each will seat 3 people excluding the driver.