Gat Outta Hell is a fun little spin-off to Saints Row 4, and is much better if you remember the whole time how cheap it is, because there's not much content to it. The 'missions' have no real variety, it's just shooting and holding buttons once in a while, so no real progression through a mission like in previous entries. Otherwise gameplay is just like SR4 which you should play before this obviously.
Graphics look quite nice, though there's no customization. Sound is nothing special and there's no radio because you're in Hell, obviously. Cutscenes take place mostly through still frames, which is annoying. Story-wise is interesting enough and we see some old SR characters but not the ones that really matter. Apart from Viola, where are the other Saints who died in the explosion? Seeing the motivations of Dane Vogel were genuinely interesting.
Most of the achievements in the game come from in-game challenges. The most tedious of them all is upgrading your weapons, which takes the bulk of your post-game time to complete. But generally, easy to get. Once you get the sin guns you'll likely use those 90% of the time as they're generally the most powerful weapon.
The superpowers you obtain here are very similar to the ones in Saints Row 4, except for summoning bad guys to help. But their helpfulness is pretty minor. Using a stomp or blast will kill enemies 80% of the the time. There is not much variety in enemy types. But you'll get sick of the shielded overlords super quick.
Once again, like in Saints Row 4, having access to super sprint and flight makes the presence of vehicles superlative. At least this time there are three diversions to do with the vehicles, which you need to do for challenges, and you should be able to fulfill the 'distance in vehicles' challenge at the same time.
It definitely is not a full game, and that shows in the challenges. Doing all of the activities and missions and collectibles takes about six hours, if that. Finishing the challenges took about four more hours. Then it took ten hours of idling in co-op to get the co-op and 20 hour achievements, which shows how pointless those achievements are if it takes double the game time to get them. And there's no real reason to play the game over again, even if you only used one character for most of it. The tiny bits of dialogue you get with the other aren't worth it.