So, it's true that the capacitive coupling of the system to ground allows some current to flow. If you connect yourself between one point in the circuit and ground in the way you describe, you become a resistor of about 1kΩ between that point and ground, and so you bring that point in the circuit to very nearly ground by virtue of flowing a current through your body. The question is, how big is that current?

It turns out that that current is precisely equal to the current flowing through the impedance between the other side of the circuit ("neutral", if you're connected to the "hot" side) and true ground. If we suppose that that impedance is purely capacitive, the question is just how big the capacitance is, because that tells us how big the impedance ((2πfC)⁻¹) is, and thus the current.

If it's 0.1 pF, the impedance at 60 Hz is 27 GΩ, and the current is 4.5 nA.

If it's 100 pF, 27 MΩ and 4.5 μA, which is still too small for you to feel. Megohms to tens of megohms is a typical resistance range for protection in the kind of neon-bulb testers that use your body to detect hot wires, because microamps are still enough to light up a neon bulb visibly.

If it's 0.1 μF, 27 kΩ and 4.5 mA. At this point you will probably yell and let go rapidly, but you will retain control over your muscles; this is still uncomfortable rather than hazardous.

If it's 1 μF, 2.7 kΩ and 45 mA. At this point your muscles will spasm and, depending on how you happen to be connected, you may be unable to let go until somebody else pulls you off. You will probably not die or suffer any permanent damage but it may be a few days before you feel okay again.

If it's over about 3 μF, and your body's resistance has become the limiting factor in the situation; for the purposes of electrical safety, it's not really any safer than if you had the generator grounded to start with.

So, the question is, how much capacitive coupling are you going to have between your gasoline generator and the actual earth, or rather, the wet concrete you're standing on? And this depends on geometry. Let's suppose the generator has a steel base, connected to its neutral wire, that is ¼m² and suspended on rubber feet of 10 mm above that same wet concrete floor. In this situation C = εA/d ≈ 220 pF, so you're down in the microamp range, three orders of magnitude away from an uncomfortable shock. If you instead have the generator on a dry wooden pallet or a dry concrete floor, you'll have about an order of magnitude less capacitive coupling. If it's on a dry wood floor elsewhere in the house and you're down in the basement standing on the wet concrete, you'll have about two orders of magnitude less capacitive coupling.

So in this situation what you need to worry about is not really the capacitive coupling; it's that maybe one of the rubber feet has a steel screw in the middle that has made conductive contact with the puddle.

Higher voltages make capacitive coupling more dangerous; you've probably seen the video of the lineman coming in on a helicopter to repair a live 100kV line, finishing by flying away while a meter-long arc connects his helicopter to the line. That's tens of milliamps of displacement current flowing across the helicopter's capacitance to the Earth, even though the helicopter is considerably further from the Earth than our hypothetical generator is from your wet concrete.

Is that helpful?