A couple people pinged me offline, so here's an update for anyone tracking it. This is a known issue, reported by other users:
http://www.machsupport.com/forum/index.php/topic,17216.0.htmlSteve explained to me what is going on: "What happens is that Mach sends a Home down to the plugin for the master axis and immediately sends a home to the slave axis. But, and here is the kicker, the Galil has the axes geared together already. So the slave axis is moving with the master at 1:1 plus the slave is getting a jog command (as a result of the second home operation) which effectively doubles the speed of the slave axis."
The home slave with master check box doesn't seem to have any effect on the issue. Hoping we'll get a plugin update from Steve when he has time to address it. If anyone else has had success in homing a Gantry with Mach & Galil, please respond to this thread. Thanks!