The meaning of abbreviations is not fixed. Different parties use different abbreviation for different meaning.
As for GM, it usually refers to General Manager. (Most likely, there will be a
General Manager in a company.)
GM usually directs the company and oversees the daily operations. In some small company, GM is like the CEO (reports only to the boss). In other firms, there could be a few GM's in a regional office alone, and their duties will be more focus on one specific area.