I currently feed my giganteas with raw grocery store shrimp that I shell myself under running tap water then freeze for use over a few weeks.
That said, IMO there is a risk of any raw food causing a bacterial problem. I've lost anemones (not giganteas) that I attribute to such issues in the past. Also, too frequent of feeding of my larger gigantea leads to its mouth being open more of the time, and heavy feeding seems to eventually lead to it expanding and contracting to the point where I was very worried about it. I stopped feeding it for a while and that stopped.
So IMO, feeding is a risky venture, that may be worth it in some circumstances, but maybe not in all. I still daily feed my smaller gigantea because I want it to grow and color up as much as possible.