It also, like all of the latter works, completely ignores the fact that going off their stated feats and power, Gohan is practically Omnipotent and Omnipresent. I mean, HOW THE HELL IS GOHAN EVER LATE!
Well, if I'm talking about the series from an analytical standpoint rather than from the perspective of a system to be used in a Quest format, the basic truth is that power consistency isn't really important to Dragon Ball.
I mean, it's important for there to be Threats, and those Threats have to be Bigger Than Anything We've Faced Before, but the series (until Battle of the Gods, anyway) had no particular inclination to increase the scale of the fights substantially after Frieza. Dragonball Z is fundamentally a martial-arts manga; drifting into the realm of galaxy-spanning energy beams, laser attacks and combat involving celestial bodies reduces the close-up grit of the fights, makes them feel less personal and the characters, by extension, less grounded.
Power levels are completely meaningless; in reality, characters are sorted into the categories of "Comic Relief", "Non-Comic Relief Unexceptional", "Exceptional Side-characters", and "Main or Temporary
Main Character". Villains are sorted into similar categories, which tend to be a half-step above their heroic counterparts. In any given scene, the abilities and skills of the highest-categorized character present will always be
Enough for the situation. They will be able to accomplish whatever task is necessary, and will almost certainly shock everybody from a lower category while doing so-- and then, maybe, be comically undercut so the audience doesn't find them unrelatable.
Which, of course, is why Gohan is always late. It doesn't matter how fast he can 'actually' fly, the important fact is he can fly there in some impressively low amount of time (a 5-hour drive distance in 10 minutes, if I recall correctly), while still being able to slip up and show a vulnerability. It's also why he can sense enemies and react to bullets fired at him, but be surprised by Videl hiding behind a locker. His strength is a conceit dictated by the plot; his strength is always "enough" to solve the current problem (because he's been established as Powerful already and is categorized accordingly as a Main Character), but not so much that it completely ungrounds him, or comes without caveats.
That said, the IC explanation of "he didn't train because it's peacetime and Gohan doesn't enjoy fighting" is logical enough. Despite their feats, DBZ characters fundamentally get their powers from fictionally exaggerated Martial Arts, and you can only keep your peak performance in real world Martial Arts through constant practice and conditioning. Slack on that for even a few years, and you'll lose a shocking amount of the ability.
Personally, I'd have liked it if Gohan actually became the main character like they were hinting all through the end of the Cell arc and beginning of the Buu one, but that was more to do with reader's obsession with Goku as the series' iconic protagonist than anything to do with Gohan himself.