First is how Goku thought both Gohan and Gotenks were required
Chapter: 514 (DBZ 320), P7.4
Context: after Vegeta has the Earth brought back with the dragonballs
Goku: “Oh, I know! You’ll bring Gohan and Gotenks back to life so they can fight.”
Vegeta: “No.”
Why would they want both of them on Kaioshin realm if one was enough? That makes no sense, the verb they used wasn't BEAT but Fight, which means both were needed to stand a chance near Pure Boo. Goku was all about Next generation and One-on-One in this arc, so it would not make sense with him to not let Gohan kill Boo if possible. If he's enough then he wouldn't want both of them on Kaioshin realm, considering how Gotenks can only maintain SSJ3 for 5 Minutes so that clearly shows Gohan wasn't enough.
First Goku called him out on how Genki Dama was not gonna work due to them taking little bits of Ki from everyone on Earth. But Vegeta asked him to take as much energy as possible, then Goku agreed with his plan :
Chapter: 514 (DBZ 320), P8.3-4
Context: after Vegeta tells Goku to make a Genki-Dama
Goku: “Hey! So it was the Genki-Dama you were thinking of!? It-it’s no use! Against Boo, no matter how much we gathered little bits of genki from all the Earthlings…”
Vegeta: “I told you, right? That those guys on Earth should take responsibility every now and then…! It won’t be just little bit. We’ll gather ki from them right up to their limits!”
Then they both agree to do it.
Vegeta told him to gather Ki right upto limit and Gohan gives his Ki, put 2 and 2 together and we get Gohan giving all of his power in Genki Dama. Gohan isn't evil to hold back in giving his power for the sake of Universe.
Goku asks to give all power :
Chapter: 514 (DBZ 320), P12.3
Goku: “Everyone! Give me as much genki as you possibly can! Please!”
After receiving his friends' Power Goku remarked on how huge it was and recognized Gohan's ki too :
Chapter: 514 (DBZ 320), P13.3-4
Context: after Gohan and co. contribute their genki to the Genki-Dama
Goku: “Oh! It’s here, it’s here! It’s already huge! This is Gohan and the others’ ki!”
Vegeta: “…Bu-but it’s not complete yet…Wh-why…?!”
Right after receiving his power Goku said it wasn't enough to wipe out Boo, despite it having Gohan's power :
Chapter: 515 (DBZ 321), P1.1
Context: after Gohan and co. contribute their genki to the Genki-Dama
Goku: “Hey, even this probably isn’t enough to wipe out Boo! What are they doing?! Hardly anyone but our friends is sending us ki!”
Goku calls them out and warned them about doom of entire universe if they refused to contribute to Genki Dama. But why will universe be in danger if Gohan is strong enough to kill Boo?
Chapter: 515 (DBZ 321)
Context : No one listening to Vegeta's and Goku's request
Goku : Hey! Everyone! Hurry up and help us! Don't you care what happens to earth and the rest of the Universe? Stupid Bastards!
I mean Gohan can kill Pure Boo even if Genki Dama fails, so why its something which can doom the Universe? Because Gohan wasn't strong enough to stop Boo that's why failing Genki Dama meant end for everyone.
After Goku releases Genki Dama, Pure Boo tries to Stop it with Kamehameha but it doesn't work despite the pushing characteristics of the wave. But after grunting, he was able to Stop the ball rushing at him with bare hands. So he definitely powered up in between that's why he was able to stop a Super Genki Dama with hands and started to push it back.
Even Elder Kaioshin said that the Genki Dama lacked enough power :
Chapter: 516 (DBZ 322), P8.5-6
Context: people on Namek and in the afterlife watch Boo stall the Genki-Dama
Elder Kaioshin: “Th-this is bad. Ma-maybe that ‘Genki-Dama’ thing st-still doesn’t have enough power…”
Dende: “Th-that can’t be…! We-we already used our ki…”
Enma Daio: “Ha-hang in there, Son Goku…! Th-that Genki-Dama has power from us here in the afterlife in it too…!”
That Genki Dama had everyone's Ki in it including Gohan's. If Gohan was strong enough they would have instantly called him to one shot him but they didn't. They all were banking on Genki Dama otherwise it was doom for everyone. If Pure Boo was weaker than Gohan then he would have been killed by Genki Dama instantly since it had Gohan's power in it.
Vegeta outright stated that next Time Pure Boo comes back it will be End of the World :
Chapter: 517 (DBZ 323), P4.6
Context: as Satan wants them to spare good Boo
Vegeta: “…Don’t you get it!? What do you intend to do if he gives birth to that terrible Boo again!? This time for sure it might really mean the end of the world! It’s best to kill him now. Got that, you idiot?!”
Why would he be willing to kill Good Boo so much? And why would it be end of the World if next time Pure Boo comes? If Gohan can take him on then he should not be scared but he was because no one was there.
Goku calms him by telling him that if Good Boo and Satan never helped them, they and everyone else would have been killed. They means Goku and Vegeta and everyone else means his family, friends and entire universe.
Chapter: 517 (DBZ 323), P5.4-5
Context: after Goku tells Dende to heal good Boo
Goku: “Well, it’s alright, ain’t it Vegeta? This Boo and Mister Satan both did well. If these two hadn’t been here, then we and everyone else would have been done in. Right?…And if worse comes to worse, we can just fight again. Let’s train so that this time for sure we won’t lose even if we go one-on-one.”
Vegeta asked Goku for solutions, Goku simply told him that they will train to be prepared for Pure Boo's potential return next time. Why would Goku's solution be training and not calling Gohan to one shot boo? Because Gohan isn't enough and they required training to be able to face him alone.
Even after 10 years he trained for him and considered him a challenge :
Chapter: 518 (DBZ 324), P10.2
Context: Goku asks Boo to rig the match-ups so he can fight Oob
Goku: “…Sorry, but I wanna fight with him no matter what, in the 1st round when both of us still have 100% of our stamina…”
Chapter: 519 (DBZ 325), P13.6
Goku: “Hey, Oob. Once your training is complete, we’ll have a match again, properly this time! [ ] To tell the truth, this is my number one objective!”