Using GaussView? I do not know. But you can draw hydrogen atoms and then modify the input file to specify isotopes.

[...]

H (iso=2) x y z

[...]

This link to the Gaussian Manual will be useful:

http://www.gaussian.com/g_tech/g_ur/m_molspec.htm

Manuel, Do you mean the Gaussian input file?

Yes, I do. Any text editor you use to modify the *.com file is ok.

Thanks Manuel, I could do it and run the program now..

https://www.youtube.com/watch?v=3F-2qp9s7_w