SPOILERS FOR AFTER THE FALL
We don't see them sign the contract, but Wesley apparently did, since he worked for them after his death, so I assume the others did too. With Fred, there was just nothing left that could keep working, her essence was (supposedly) destroyed.
And in a way, Gunn did do as the senior partners said(if I remember correctly), or at least what they wanted. But either way, he became a vampire, he didn't really die, so his "soul" or whatever couldn't be tied to the senior partners(or maybe his soul was off working for the senior partners while he was a vamp). Point is, I think it might just be the true dead, like Wesley, that counts, for the clause to work. That's my guess.