In 15mm scale we game the period mainly using Battlefront's excellent "Team Yankee" rules, while in 28mm we've used several sets as bases for our gaming, including but not limited to "Bolt Action", our own set ("Red Storm!"), and even a variation the venerable "Lord of the Rings" skirmish set...
Eureka US Army soldiers in MOPP suits.
Some reference material... there are a couple pics of M109s in the Reforger book as well as some detail of the other markings. Similar to the M1 Abrams and M113s I've painted, I didn't put national insignia on the M109, but instead limited markings to an "exercise" marking as used in the Reforger exercises.
I'm pretty happy with how the build turned out and I think it'll be fun to have on the table in a game. Of course the 155mm main gun is not really suitable for tabletop action in 28mm but we'll figure something out!