Gallo is unlocked by obtaining the Infinite Corridor Item and Divano is unlocked by obtaining the Crimson Shroud Item. Some light spoilers ahead so stop reading if you want to continue a blind playthrough of the game:
In order to unlock either of these characters you need to do the following steps in order:
- Unlock Hyper Mode by defeating the 25-minute boss in all 4 normal stages
- Survive 14 minutes in moongolow to get teleported to the Holy Forbidden Area
- Grab the Yellow Sign Relic at the end of the Holy Forbidden
- Replay any of the 4 stages and you'll notice 4 new stage items: 2 blood-red arrows and 2 rings.
- Either collect both rings while having the clock lancet or collect both arrows while having the Laurel.
- Upgrade all 3 (or all 6 if you want to kill the Grim Reaper at the end of the stage) and then evolve them with a Elite Enemy chest.
- After evolving the Lancet you will get the Infinite Corridor.
- After evolving the Laurel you will get the Crimson Shroud.
The Yellow Sign can actually be unlocked early by using the Forbidden Scrolls of Morbane and by casting the Spell "ihaveseenitihaveseenitihaveseenitihaveseenitihaveseenitihaveseenitihaveseenit". Divano can be unlocked with the spell "quandolodicelui" and Gallo can be unlocked with the spell "waitreaction" Although I am uncertain as to whether this affects the unlock for the associated achievement.
Lastly I'd recommend grabbing both characters at the same time as the two upgraded weapons are required to kill the Grim Reaper at the 30-minute mark. Doing this on Mad Forest will also get you the Randomazzo perk: Mad Groove which pulls ALL items on stage to you every 2 minutes, great for speeding up your other grinds :)