There's build up to all of those moments though. It's not a Deus Ex Machina if there's a sufficient amount of groundwork laid out Gohan's "hidden potential" to kick in. I guess the tail growing back during Vegeta's fight counts?
And hey, I don't think Goku's one either! Most of the time anyway.
There was no build up for Gohan's first rage out or Gohan randomly regrowing his tail. Old Kai poping out of nowhere with his abilities also has no build up, it just happens.
While Deus Ex Machina may not apply to his other rage moments or his 'hidden potential', Gohan comes off as both cheap and a plot device more than Goku since we know Goku always train, while has been handed several free power-ups with Ultimate Gohan being the worse. Even his boost in Super, while he did train for over two years, feels hollow because he got most of his boost in hours.
As for Trunks, I can't see him as a Deus Ex Machina since Goku would have owned Frieza himself, so Trunks was unneeded.