Okay! I meant to do this months ago and then became paralyzed with indecision about my picks, but no more! I'm forging ahead! I have read at least two things from every page of the kkg ao3 archive, and probably up to 80% of the archive total, so I can give you the distillation. I haven't included any of my own fanfic or Neveralarch's, since if you follow me you'd probably already know about those.
The Essential Kakashi/Gai Reader's Guide
An intro to the KaKaGai ship (being fanfic that is short, straight forward, and forms a multifaceted overview):
āFeverā (rival shipping but longing flavor)
Shiver (tragic but horny flavor)
āSplendid Displayā (established relationship, explicit)
to build a dream on (happy fic, childhood friends-to-lovers)
Guy's Choice (FAVE. "The problem with being a hero is that some bastard will come along and force you to choose between your life and your loverās. Guy makes his choice.")
Leaves After the Fall (The kind of sticky emotional misunderstanding climax I eat for breakfast)
Only Ever You (high drama, mutual misunderstandings)
New Year's and Forever (happy fic, retrospective structure)
Family's Just Another Word For Something Left To Lose (reels you in with found family, slams you with realistic childcare hurdles, and then pulverizes your stomach with last minute romantic drama & PTSD fallout. Chapter 13 concludes the main storyline.)
Boudoir (Sukea identity + horny)
Everything That Touches UsĀ (gentle romance arc)
Childhood Not-Friends (12 year old Kakashi is so fucking stupid)
Some of the crunchy chewy check the tags first kind of fic fic that I personally like:
Resisting the Relative Motion (awkward teenagers + sex)
Stone Avalanche (Polished up RP fic; great sense of character, it is both charmingly juvenile in places and absolutely brutal in others. Basically, a perfect tonal match to the Original Goods.)
selfish enough to burn (Kakashi does a time travel fix it AFTER watching Gai cook himself on the 8th gate. His relationship with their younger selves gets messy)
history has its eyes on you (Brutal suffering on the first half; complete and utter (difficult) recovery on the back half. Kakashi does childhood a second time but he really really needed it.)