Check engine light code 0300

P0300 is for random multiple misfires. If you have an advance scanner I would look at the misfire counter if available and see which cylinder is misfiring the most to try and pinpoint the exact cylinder/cylinders misfiring.

GM uses a spider type injection system which has had a lot of problems in the past. Usually the fuel pressure regulator or the injectors leak causing the random misfires. Attached image with the regulator circled.

The cylinder misfiring the most check for spark. If you have good spark then do a compression test on that cylinder and on a cylinder not misfiring and compare readings. If they are pretty close then replace the fuel injector assembly. The new assembly comes with new injectors and fuel pressure regulator. You will need to remove the intake manifold to get to the spider injection assembly.

You may ask why it is called a spider? When you remove it you will see why but attached an image. Let us know what you find with the quick tests mentioned.