Having been through this twice -- at the elementary and magnet levels -- can I just say that it's really too early to return on the basis of missing friends. It's only been a little more than 1 quarter. It's normal to still miss friends, because new friendships haven't really been solidly formed yet.
I would advise to try to build bonds with the HGC students. Try to do "playdates" even though they are in 4th now. Build on the holiday season by inviting small groups of kids to do fun stuff w/ down/talking time (like pizza night, movie afternoon, holiday cookie making, museum visists, whatever).
The new friendships my child built at the center are still critical for her. Those kids "got" each other in a way that their friends at the home school never did. I don't think my daughter realized that until the end of her first year.
Also, I tried to emphasize that friendships wax and wane our whole lives. No one stays the same or in the same place anymore. Try to emphasize that the circle is expanding and has more shades of friendship now.