Except Equinox doesn't taste of grapefruit. So no amount of Equinox will result in a grapefruit flavored beer.
Equinox is often described as papaya and green pepper. I'm not sure that accurate but its closer than grapefruit as a descriptor
Also there is nothing stopping you increasing you hop additions to compensate for a lower total oil content/g.
I disagree with a lot of the hops suggested in this thread.
Most hop taste of a generic citrus amongst other flavors. Even for those that are genuinely grapefruity that's only part of the story.
For example ctz is grapefruity but it is also very resinious and dank. Simcoe is grapefruity but also piney and catty.
Citra and galaxy are not particularly grapefruity. Citra is primary mango and tropic citrus like pineapple and galaxy is a more complex mix of fruit flavors.
Amarillo is often described as tangenalo which I think is actually an accurate description.
Centennial is very grapefruity but also very floral.
Cascade is quite grapefruity, so I agree with that.
Now I not saying you cant get grapefruit from these hops, after all the final flavor is a combination of ingredients rather that simply stating Citra will give you grapefruit isn't true.