One possibility is:

Google for "nanotube filetype:pdb"

Save the file and then open it in Gaussview or use a software for converting it from pdb to gjf (OpenBabel, for instance).

Good luck!

You can use this website for generating any kind of CNTs you want, with a wide variety of chirality, length and diameter

http://turin.nss.udel.edu/research/tubegenonline.html

The out put file will be in the form of .gjf

I'm not so sure whether you can use this modeling in DFT calculation using Gaussian 09 , but it is worth to try 

Please don't forget to cite them, based on their request 

Good Luck !

 I have made a simple App for making CNT gjf files. (However, it only works on macOS, not windows).

If you are interested in it, please contact me and I will send you a copy of it. 

Best,