Daisy Buchanan was unable to attend because her husband Tom and she had packed up and left the area without leaving any information about where they were going or how they could be reached.
Meyer Wolfsheim wrote to tell Nick of his shock at hearing the news of Gatsby's death but explained the he was "tied up in some very important business and cannot get mixed up in this thing now." A subsequent phone call from a mutual acquaintance of Gatsby and Wolfsheim reports that another individual, Parke, has just been arrested. Wolfsheim cared for Gatsby only as he could help with his illegal activities.
Mr. Klipspringer boarded with Gatsby for a time but never intended for their relationship to be one of anything other than convenience. He was staying with friends in another part of the state at the time of Gatsby's death and couldn't be bothered to change his plans and come back for the funeral.
As Nick discovered, Gatsby had many superficial relationships but very few based on true friendship.