What would giants want? Herds of cattle, bison, or alpacas to turn into food and clothing. Going back to Gonn Orta and Boshbisil, they want magic even more than meat, but they also want help with "fine manipulation," little hands to thread needles (or harpoons), artificers to stitch sailcloth garments, fashion locks, carve scrimshaw and repair pea-shooters. These could be slaves or skilled craftsmen (slave-craftsmen?). Giants would appreciate being able to peer at little stuff better, so even if your Glorantha does not include magnifying lenses and telescopes, they might go for hemispheric lumps of polished glass.
As what humans want from giants, the record indicates construction services, quarried stone, building walls around Pavis and Asgard, or hexagonal causeways. Giants are good porters until they get hungry or distracted. Or maybe the giants have a domesticated mastodon or apatosaurus they want to sell?