I've been involved in the design and installation of a number of Natural Gas fired standby generators for various facilities. They are piped directly to the NG distribution system. I have not heard of a failure from one of them, but of course they get little run time, mainly monthly test runs and the very infrequent power outage. I believe they used Chevy V8 engines appropriately modified, and have no provisions to switch to gasoline, although a couple also have an LPG tank and automatic switchover system if the NG system experiences an outage. They do the job they were designed to do.
