They're two different time zones but there's not much variation between the two. Central European Time (CET) is 2 hours ahead of Greenwich Mean Time (GMT).
However, temember that CET is not always on because during the summer, locations on CET are set on Central European Summer time (CEST). This means that 1 hour is added to the usual CET time.
GMT doesn't vary because it an absolute time reference and doesn't change with the seasons. However, some of the areas that are located in that time set (like the UK or Ireland) switch to Daylight Saving Time, British Summer Time (BST) or Irish Standard Time (IST), always adding one hour.
For examples, cities like Reykjavik are on GMT all of year, whereas cities like London are not. I hope this doesn't sound too confusing!