The effect of Thalia and The Gitrog Monster’s ability that allows you to play an additional land is cumulative with similar effects. For example, if you control Thalia and The Gitrog Monster and another permanent with that ability, you’ll be able to play three lands during each of your turns.

As the last ability resolves, sacrificing a creature or land isn’t optional. If you control a creature or land (and if you control Thalia and The Gitrog Monster, you probably do), you must sacrifice one.