Close Friends?

I don't think I would call Nick and Gatsby "close friends." Gatsby was probably the best friend Nick had at this time in his life, but I wouldn't consider them close. To me, close friends know and accept each others deepest and darkest secrets. Nick didn't truly know that much about Gatsby and the things he did know he questioned. They may have been friends and neighbors but I don't think they could be called "close friends."
I think there are a couple different reasons he tried so hard to get others to attend the funeral. For one, he realized Gatsby didn't have any true family and friends around and he felt responsible to try to honor his dead neighbor. Even Nick with all his problems and character flaws, didn't want another human being to have a funeral without the proper respects.
The other reason I feel he tried so hard to get people to attend is that he didn't want to accept the reality that Gatsby wasn't the person he thought he was. Nick didn't want to realize this obsession he had was all in vain. In a way he is very similar to Gatsby in that for so long he was so obsessed with something that he couldn't actually have. Gatsby was obsessed with and wanted Daisy, and Nick was obsessed with Gatsby and wanted his life. Both of them wasted a good portion of their lives chasing things that continued to evade them. In the end they should of realized what was really happening and accepted the truth.