Yeah, just like Yamma said, this is the level of attack that the poisoned victim has to make an RR against.
I believe the range for G is -10 to +17. You get this from the creature variable level chart. If you roll on the chart, you get a modifier to the level of the poison; or you can just say it's 3 (from your example) and ignore the G. But with the chart you could have a level 1 poison (1 single bee sting) to a level 18 poison (tougher than a Komodo dragon).
Even if you use the variable and roll high, the poison could be reduced per successful attack the creature makes or whatever you decide.