For this achievement all you need to do is Climb the old goat mountain castle from the outside and then jump into the magic sphere to unlock the gravity goat mutator, The castle can be found by following the map north then cross the bridge on the west over the lake, then follow the path round by going left after the bridge and you will eventually find it. If you having trouble finding it look to the sky and there is a big red beam shooting into the sky. Once you are at the castle go to the entrance and there should be a lava pit, jump into it and bounce about 3-4 times which should get you the height to move onto the next area where you need to repeat the process again. Once your at the top you will see a big purple sphere if you jump into that the gravity goat mutator will unlock and your achievement will pop
This is one of the simplest 90G in almost any game, If you would like a guide of how to find the castle or a video guide for the whole achievement I have included my guide below