The earliest you can travel to Gunner's Heaven (it's a sort-of town) is when you get the sandcraft (you get the sandcraft automatically through the story).
Travel via the sandcraft to the very top left of the world map. There is a beach there you can dock at. Now travel south via foot/horse until you're a little above the middle left of the world map. Scan around there and Gunner's Heaven should appear.
You must be level 25 to challenge the Novice Division but you should already be that by the time you get the sandcraft. You'll be tasked with fighting 5 battles which must be completed in 25 turns (not per battle, all 5!).
The first battle is likely the longest because it's 3 enemies with a lot of HP. I'd recommend using your summons here to damage all 3 enemies. The next 4 battles are much easier and mostly involve single enemies. Also note that buffs persist through battles, including debuffs/status ailments. I would recommend bringing a few bronze-curative items.
Small tip - put the light guardian on Gallows. Don't heal him at any point (he should have lots of HP anyway) and let him take damage. Cast the arcana "Valiant" - it will boost his damage based on how much HP he has lost. If you combine that with gatling then he can take out an enemy in just one turn! His normal attack will also be really powerful. Valiant will persist between battles too so no need to recast it.