I agree with Wolk. Nova blast is an ability exclusive to a Toa. Even Ignika needed to create a Toa body for itself to perform a Nova blast. That is not to say it could not have achieved the results of a hypothetical life nova by itself (Greg confirmed it could), but a true actual Nova blast was possible thanks to a Toa body.
One more thing - based on some other decisions and limitations placed by Greg, I highly doubt he would allow any elemental Kanohi to be able to do Nova blasts. He specifically disallowed Mask of Elemental energy to recharge you to a full tank after a Nova blast. Also, elemental Kanohi work slightly different from a Toa, despite giving you Toa-level elemental powers.
As I explain in my Many flavors of energy in the Matoran Universe topic, masks have their own separate energy source powering the mask abilities. In case of elemental masks, this has to be converted into elemental energy. But as I describe, all masks have an absolutely massive energy capacity - so much so that the oldest masks in existence didn’t diminish in power even after 101 001 years of heavy use. Nova blasts of the Toa are balanced by the fact that the elemental energy tank has limited capacity and it needs to be recharged after going nova, hence the Mask of EE limitation. If elemental Kanohi allowe for Nova blasts, one wouldn’t need to wait and recharge, they could just keep on blasting until the mask runs out of energy many years later.
Additional piece of trivia/observation: the reason elemental Kanohi don’t allow you to absorb elements is once again tied to the functionality difference - it is said the Kanohi cannot be recharged ever. So it makes perfect sense that elemental Kanohi cannot reverse-convert elemental energy into mask energy and hold onto it.
And to answer the original question, while it is true as others pointed out that GSR had failsafes against planet-killing attacks from within (such as the domes being able to isolate and restrict Nova blasts), if someone has knowledge of critical GSR systems they could certianly use Garai or any other power to damage it. We look no further than Hewkii lifting up the entire Coliseum with his Garai. In Dwellers in Darkness he had to slowly put it back as to not damage anything. But what if he lost his mind and decided to shove Coliseum straight into the core processor situated right below it? GSR would get an instant stroke and die.
So yes, it is possible to control gravity via Garai in ways that would damage GSR.