The need for Ice is nonexistent before Pyrus mountain. You will have access to many things by then. What most people do is keep several pokemon as backups, or on rotation. These can be from 3,4 or complete teams, according to how you want to play (check the signature for an example). Try to keep things ready for many different sets of events.
In my opinion, instead of a Gigalith, I'd rather have an Excadrill. It has similar bulk, less weaknesses and can have a perfect neutral coverage with ground and rock moves (plus steel as STAB), but that's up to you.