@Simonmoon is exactly right >>
It is also my opinion that they lost a lot more, musically speaking, when Steve Hackett left the band, than when Gabriel left the band.
For me, it is not the Gabriel era vs the Collins era. It is the Philips/Hackett era vs the post Hackett era.
Hackett and Philips both did solo albums that are much better that any Genesis album of the post hackett era.