When used, K. Rool will puff out his stomach whilst a green visual effect is displayed on the armor, and reflect any damage onto the opponent. It works like a counterattack and can also reflect projectiles. Despite its appearance, the move does not interact with K. Rool's Belly Super Armor in any way; countering an attack does not deal any damage to his armor, and he can counter attacks that would normally break his armor in one hit (such as a Falcon Punch) even if the armor has already been damaged.
Gut Check has a damage multiplier of 1.5× for both counterattacks and reflected projectiles while dealing 12% minimum, making it second after Joker's Tetrakarn for the highest damage multiplier of any counterattack, thus being a rather strong counter. This allows some countered moves to send an opponent to kill percent. It also has surprisingly long periods of intangibility on the counter (frame 4-15), allowing attacks or grabs to miss K.Rool entirely, though this is of niche use due to the move's high lag.
Gut Check's counterattack/reflection can actually be reversed (although not the counter preparation itself), with the reflector still being active until the move ends. This makes it a useful tech choice in Team Battles or free-for-alls due to the counterattack's intangibility, giving him additional defense against oppressive projectiles or aggressive opponents. It is also handy as a mixup if the opponent would otherwise be stage spiked, although this is mostly a gimmick.
However, the move has several downsides. The counter hitbox is centered on K. Rool's stomach, making the rest of his body vulnerable, much like the Melee and Brawl versions of Toad. This means opponents hitting his head or feet will not trigger the counter even if they face him. Additionally, Gut Check has heavy ending lag (34 frames), making it one of the slowest and most punishable in the game, alongside Palutena's. The move is also notorious for having a laggy counterattack animation; despite its frame data buffs in update 3.0.0, it doesn't do well against projectiles with low cooldown, as it has enough ending lag for an opponent to punish K.Rool if they dodge the reflected projectile. It is also possible to reflect a reflected projectile, usually resulting in K. Rool being forced to take a hit while stuck in the counterattack's endlag. Gut Check can simply miss or hit with the sourspot (on the "gust of wind"), which makes it unsafe against some ranged fighters.
The move is best used as a combo breaker or on reaction, in order to prevent K.Rool from receiving a punish. It is also handy as a ledge option, as jumping with it halts all aerial momentum. While this can severely hinder horizontal recovery, it can work as an effective fake out that's faster than simply climbing up; additionally, since the counter starts on frame 5 (with frame 4 intangibility), it is actually one of K. Rool's fastest ledge options. While on stage, this can also be used to punish getup attacks or opponent's ledge options.
If an overstrong projectile breaks through the move's reflector, it will still trigger the counterattack.
This move is likely inspired by several instances of Kremling resilience. K. Rool himself is seen using his big frame to attack by bouncing on his stomach in Donkey Kong Land; this also inspired his dash attack and neutral aerial. It could also draw inspiration from King K. Rool's immunity to front attacks in Donkey Kong Country, which is due to his belly being plated in armor. Throughout many games in the Donkey Kong Country series, Kremlings other than K. Rool who took attacks to protected areas of their bodies (or were attacked with insufficient force) could shrug off the blow, and in some cases damage their assailant. In Donkey Kong 64, Klump was a tough enemy who damaged Kongs who made contact with him by bulging out his belly in a manner most similar to Gut Check.