Funny... I am actually working in a few One-shot lemons involving Gohan with girls from both DBZ and other series. Esxamples include:
From Dragon Ball/Z/Kai/GT/AF/whatever-the-hell-else:
Gohan/Bulma
Gohan/Chi-chi (This is during the 7 years after Cell, though the first time is a Gohan/Chi-chi/Goku three-way)
Gohan/Pan (Involving Pan Time-traveling from her world as the battle with Omega Shenron goes south, so both are the same age)
From Elfen Lied:
Gohan/Kaede (Also known as Lucy or Nyuu, from Elfen Lied. Primarily the Nyuu personality)
Gohan/Nana
Gohan/Mayu
Gohan/Yuka (Yes, she gets to screw him too, problem?)
Gohan/Nozomi
Gohan/Mariko (But she's a teenager here, remember, the Silpelits age faster)
From Madoka Magica:
Gohan/Kyoko
From Magical Girl Lyrical Nanoha:
Gohan/Fate (Sorry NanoFate fans, but I couldn't resist)
From Pokemon:
Gohan/Misty
Gohan/May
Gohan/Dawn
From Negima:
Gohan/Asuna
Gohan/Konoka
Gohan/Chao
Gohan/Yuuna
Gohan/Most of the other girls in Negi's class
Others:
Gohan/Arisa (From the Hentai Ova "Moke Moke Mutsume Arisa")
I could go on and on, but this is about it for now.
As a final detail: Gohan in most of these lemons is essentially the form known as "Teen Gohan".