Super Saiyan said:
Why was Goku so confident in Gohan’s victory beforehand? Surely he couldn’t predict that Base Vegeta would distract Cell?
Chapter: 458 (DBZ 264), P4.4-6
Goku: “Ah! Wait, Gohan. I have 2 more senzu; you should eat 1. You used up a lot of stamina earlier. [ ] Get angry, Gohan…Remember how you got angry and fought Cell, and draw out all of the power you have. If you do that, you won’t lose to anyone in the entire world! Not to anyone…
Goku clearly had faith that Gohan's rage would allow him to surpass anyone, particularly noteworthy when Gohan's peak at that point had already been surpassed by Goku (and far surpassed as we'd see later on). If Goku had so much faith then despite the gap, I don't see why he wouldn't against Cell.
GreatSaiyaman123 said:
Cell surely wouldn’t have been atomized out of existence when overwhelmed with a blast weaker than his own.
I don't think anyone's saying it wasn't. The Daizenshuu makes it pretty clear Gohan got a rage boost at the end of the beam struggle, as do several statements from the Boo Arc and implications of its scaling.
And he wasn’t horsing around with Gohan either.
Of course he's not horsing around if it's not BoJack.
I wouldn't exactly say this is the face of someone taking things entirely serious.