This is a question that I don't have the answer to. I'm very unfamiliar with G-scale or what sort of keep-alives are even available for large scale. Whatever you use should be capable of handling the power that a LM-3SG can deliver. The specs for the LM-3SG are available on the LM-3SG instruction sheet, available from Ring Engineering's website.
So you don't have to go find them, here they are (though I do recommend looking through it anyways)
Specifications
Maximum Power Input Voltage: 24 Volts
Maximum Motor Stall Current: 8 Amps
Maximum Output Current on a Single Output: 1.2 Amps.
Maximum Output Current on all Outputs Combined: 1.2 Amps.
Audio Output Power: 13 Watts into a 4 ohm load. 7.4 Watts into an 8 ohm load
-Kevin