how to comment block of codes in Vim & gVim

Put your cursor on the first # character, press Ctrl V (or Ctrl Q for gVim), and go down until the last commented line and press x, that will delete all the # characters vertically.

For commenting a block of text is almost the same: First, go to the first line you want to comment, press Ctrl V, and select until the last line. Second, press Shift Esc (then give it a second), and it will insert a # character on all selected lines. For the stripped-down version of vim shipped with debian/ubuntu by default, type : s/^/# in the second step instead.