Until you actually get citizenship, quitting at any time is a risk ... but at some point (3 months? 6 months? 1 year? nobody knows, until there is a court case) the risk becomes less than the chance of being struck by lightning.
Was this job the only job you ever had in the US? You don't have to do anything special just because you have a GC. No need to tell USCIS anything. The usual job-leaving steps apply -- get an experience letter, make sure they have paid you for any unused vacation days, and reimbursed you for job-related reimbursable expenses. When you resign, don't disparage the company in your resignation letter, even though you might hate them. And out of courtesy you are expected to give at least two weeks notice even though it is "at will" employment.
If you have an upcoming bonus (but will leave before the bonus is paid) make sure they agree to pay you the bonus if you have stayed past the date to be eligible for it. For example, at my company I have to stay until December 31 to be eligible for a bonus that will be paid in March, but if I leave after December 31 and before the bonus is paid, I am still supposed to get the bonus according to the policy.