Since when has a timezone been called a gmt? GMT is The timezone of britain, iceland, portugal, morocco, ghana, and anything else near that line. Columbia is probobly -5 off the top of my head.
Does it metter? He asked what GMT difference Columbia is in & preferably wanted an accurate answer, some people work in GMT, some in UTC & whatever other timescales there are.
Its not that I didn't believe you, I just wanted to show you that Columbia does obviously come under a GMT timezone, even if it isn't its main timezone unit