Do you need mass equal to earth, or mass slightly greater than whatever you're launching? (Alternatively, does antimatter experience the opposite force of the s

Do you need mass equal to earth, or mass slightly greater than whatever you're launching? (Alternatively, does antimatter experience the opposite force of the same mass of regular matter, or is it something else?) If antimatter has negative mass in the context of the gravitational equation (F = Gm_1m_2/r^2) then you just need more antimatter than matter (by mass). Of course, that also means general relativity is incomplete. That's not to say launching/lifting something on an antimatter platform would be practical anyway. There'd be issues around how you keep the platform and payload together, let alone the difficulty of constructing an antimatter platform in the first place...