JoeLucid might have some ideas, but the thing that comes to my mind is that if all your nodes are in lowpower listen mode and the Gateway is in some kind of timed sleep mode (either listenMode timer, RTC, or WDT timer) then, when the Gateway wakes up to gather samples, it simply sends a wakeup burst to the nodes, the nodes wakeup and, on a well defined response time based on nodeId or somesuch, each node sequentially reports their samples to the Gateway.
This would be a low power alternative to the Gateway being on all the time, however, the burst (depending on your operating parameters) could be as long as 3 seconds and then, with the Gateway constantly receiving until it has received all the samples, the Gateway power consumption would be relatively high for several seconds which would quickly drain a low mAH battery.
An alternative is to simply deploy a dedicated Gateway that runs off the mains and gathers all samples from your other nodes as well as the Moteino you just deployed.
Good luck and have fun!
Tom