Garnichts is a strong, emphatic way to say "nothing at all" or "absolutely nothing" in German. It's more intense than simply nichts (nothing).
In the context of "Ohne Dich," the line "Weil ich ohne dich ein Garnichts bin" (Because without you I am absolutely nothing) powerfully conveys the singer's complete dependence and sense of emptiness without the beloved. It's a dramatic and memorable expression of profound loss.
The title "Ohne Dich" translates to "Without You," and this song is a powerful exploration of what that means. The lyrics paint a picture of a world that is incomplete and meaningless without a specific person. The singer uses a series of strong comparisons to make his point. Just as there is no harvest without a seed, no faith without hope, and no light in the darkness, the singer feels there is no warmth, love, or even meaning in a world where this person isn't present.
The chorus drives this feeling home, stating that without this person, everything is a "schaler Traum" (a stale or tasteless dream) and life is "ohne Sinn" (without meaning). The singer feels they are a "Garnichts" (a nothing) and that all their time is just "verlor'ne Zeit" (lost time) without their loved one. It's a dramatic and emotional way of saying, "You are my everything."