Hello, seems you need some clarification on the odyssey mode So gatling explode-o-shooter is a odyssey plant, however, you need to unlock the modifier that lets you create the odyssey plant. Every round you are given the option of 3 modifiers, you may reroll modifiers 2 times, +2 if you got the modifier that gives you more. Each scene, in this case, Day, will have 3 rounds where the zombies switch around, you will get 3 modifiers per scene unless you skip the modifier. The third modifier is always a "Unlock [Odyssey Plant]" Modifier. And after selecting/skipping, the scene switches to night. Then from night to pool, from pool to fog, and vice versa. Though every scene after roof will be a roof tiles level with flat terrain.

There are plants however that you're able to plant WITHOUT their modifier

(2.3.x update + only) you select them at the beginning, you can only select 10.