The employer can not revoke it (that woudl be CIS' task), but the employer can of course complain to CIS if he thinks that you just used him to get the GC. CIS could then investigate the matter, and at the end of such an investigation could be a revocation of the GC.
At the time of becoming a PR, you have to have the good faith intent to work for the employer. Changing jobs soon after getting the GC puts that in doubt, and in case CIS investigates, the burden is on you to show that you had the required intent.
See the numerous threads on that topic here.