I suppose we ought to be relying as much as possible on real-world biology and stuff here, since we're not using "handwaving" or magic to take care of the problem.
The giants seems like a mammal, and, of the mammals, seems most closely to resemble the primates. Of the primates, most giants seem to resemble humans and thus are probably similar to the other great apes (gorilla, chimp, orangutan and bonobo, if memory serves).
The gorilla, the largest of these, eats mostly plants. The chimp is somewhat omnivorous, but most of its meat comes from bugs and monkeys--otherwise, it too has a high vegetable-content diet.
Humans can subsist on mostly plant matter, but meat (from bugs to fish to larger animals) seems to have always been a supplement to the human diet.
If giants are relatively low in number (i.e. there aren't many giant cities), then they could probably get by eating off the land, as it were, or by raiding other communities. If you have whole communities of giants, I'd think they'd require a rather well-cared-for system of agriculture in order to survive.
Dave