Setting the column gap in a twocolumn (or multicol) document

Use \setlength{\columnsep}{<width>} in the preamble of your document, i.e. before \begin{document}.

Changing \columnsep also works if you use the multicol package.

As meteors mentioned in his comment, \setlength{\columnsep}{5 in} also did not work for me. I was using \twocolumn command. Instead of separating the two column, I adjusted the widths of each column and it did work out well i.e.,

\setlength{\columnwidth}{<width>}

I ran into this problem earlier today. As Mauz Khan and Evan Aad note, the solution of adding \setlength{\columnsep}{<width>} to the preamble or in the body (prior to starting the multicol environment) does not always work. If you are in certain modes (Beamer, for instance) where \usepackage{multicol} is being called in your .sty file (rather that in the preamble of your actual document), modifying \columnsep in your document's preamble will not work. In this case, you would need to modify the length in the preamble of the .sty file by adding \setlength{\columnsep}{<width>} there instead, rather than in your document.

I hope this helps someone else! Perhaps its more of a comment, but this is what I am limited to.