。
At least answer me, come on.
Literal
Reply at-least do [assertion-よ].
ぐらい after 返事 sets a minimal expectation — 'at least a reply.' しろ is the plain imperative of する, blunt and masculine. The よ adds exasperation rather than mere assertion, turning a bare command into a frustrated demand. This is how you'd scold someone for ignoring you.