too bad the site doesn't help because it only lists one room and gravity boost is necessary in more than one room. m2k2 only covers in-bounds breaks, all I remember about gravity boost is you need to use secret worlds to skip it though exactly how I don't know and the wiki's not working so *shrug*. If you really need to know you could always check DJGrenola's low% on SDA.
Yes, it's segment 10 in the 17%. It's been a while, but it goes something like:
i) SW yourself the early power bomb expansion off Temple Assembly Site ii) use underwater dashes in Hydrodynamo to access the doors; do not open up the bottom of Hydrodynamo, since you'll need to use the cannon to get back out of the water, and it disappears if you open everything up iii) use the early power bomb to pop the power bomb door in dark Hydrodynamo (real name escapes me) and recover the Chykka key without using the nearby portal iv) return to light world the way you came and use the cannon to get out of Hydrodynamo Station.
Should you experience some trouble with the underwater dash, a substantially easier alternative is available. Take the Gathering Hall portal, instead, and ghetto jump up the laser in Crypt. Power Bomb the door to the Undertemple Access(?), grab the key, and head back through the Crypt portal. It should be noted, however, that if you're interested in also skipping the Grapple Beam, the underwater dash in Catacombs is preferable. Also, the route through Crypt is longer.