。
This is all I can do for you.
Literal
I [subject-が] you [to-に] can-do thing [topic-は] this only is.
僕が君にできること is a relative clause: '[the thing that] I can do for you.' できる as a potential verb takes a が-marked ability. これだけ ('just this') is an exclusive quantifier. A humble acknowledgment of the speaker's limitations and gift simultaneously.