I have no experiance with gigantea anemones, but I have had an H.magnifica for the last 6 months. I dont know how well my experiances apply, but I was told by alot of people, (including one friend who's kept the same H.mag for the last 11 years), that clownfish will help the anemone adjust to tank life. No scientific reason given why....but through anecdotal evidence, the H.mags at least, do better with clowns in attendance.
2nd, my H.mag had been in captivity for several months under 20K mh's and went to 10K's in my tank. My lights are suspended over the tank, so I raised them as high as possible off the water surface, (18 inches), to lessen the intensity.
The first week the anemone would deflate and look like hell at least once or twice a day. I was convinced it was dying and get frustrated. A couple of hours later, the anemone would be fully inflated, and look fine. This slowly stopped happening over a three week period.
My hypothesis is that the light was more intense than it was used to, so it would deflate in order to decrease its surface area available for photosynthesis.
Or, it could be the anemone was just reacting to the stress of being bagged up and shipped somewhere.
My point is this:
If the anemone is attached, the tentacles are sticky and reactive to feeding stimulis, and the mouth is tight, you're still in the game.
I wouldnt necessarily feed the anemone all that often, becuase you can stress it out by feeding so much.
Just work at keeping your tank parameters in good order, and I think your anemone will do fine.
Nick