Grid current is a small DC current, coming out of the grid. If you have 1uA and a grid resistor of 2mOhm, that means 2 Volt DC. With a 300B that means 10mA (about 12%) more plate current as you intended.

A good tube has only a few hundred nA grid current only. But how good is a random tube, and why can't you use it any more just because a good working tube has some grid current. The advantage of a grid choke over a grid resistor is the perfect protection of the tube against grid current, coming either out of the tube itself, or it is generated by overdriving the tube. Both ways for getting grid current can happen, and the best protection for expensive tube is a grid choke in that case.

Some also say a grid choke simply sounds better. Here is what causes this: A grid choke increases the input impedance of the tube it is connected to. Input impedance is a several Megaohms or more, with a grid choke. This reduces the distortion of the driving stage significantly.

A not so well known application is the 1:2 step up option with the LL1670. So use it as a auto transformer and as a grid choke at the same time. That gives a noise-free gain of 2x. It reduces the Inductance to 405 Henry, but that is more than you need still.