I'd say 3/0 gravity pull is just as useless as 3/0 devour
. Of course any ability on a 0 hp creature is almost useless... the only way its useful is if you use the mutate on an already established creature and its ability is growth or something. 3/0 devour is actually technically better than 3/0 grav pull because of the fact that if you have something like a blessiing, the devour creature will actually be useful whereas the grav pull one will still take just 1 hit to kill
.