Sorry, it is quite possible the mode is listed as "manual" and not custom. I am going off memory here (and my memory is terrible)!
Here are a couple of shutter speed decimal equivalents.....
1/2000 = .5 ms or .0005 seconds
1/1000 = 1ms or .001 second
1/500 = 2ms or .002 second
1/333 = 3ms or .003 second
1/250 = 4ms or .004 second
Please note, you can also set the shutter speed as a range from fastest (0 ms) to the desired slowest shutter speed. So entering 0-4ms will allow the cameras to select any shutter speed 1/250th or faster (so 1/500th or 1/1000th, etc). I generally set a range instead of demanding a single shutter speed be used (although honestly it probably doesn't make a difference in real life). The slowest shutter speed will be different for everyone, but likely needs to be 2ms (1/500th) or faster to ensure images have no motion blurring in them. If you have fast traffic you might need to increase or if the traffic is slow (for example I'm at the end of a street in a cul-de-sac, so traffic cannot go fast past my house) you might be able to use a slower shutter speed. I'd recommend you use 0-2ms to start and see how that works.
The gain setting will appear as a range with the max probably at the default "50" (out of 100). That isn't a bad number to use, but you can adjust it higher if you need the brightness to increase because you plates are too dark to read, or lower if the plates are still getting over exposed and you don't want to change the shutter speed. It usually takes some playing around with both the gain and shutter speed values before you land on the "perfect" compromise that allows motionless captures that are bright enough to be useful.