The use short form in case of Company names in RAP, SLS, presentation?

It is perfectly acceptable to use GM most of the time in your report as long as you have given the company full title beforehand as the company is often known as GM (and for VW for example the abbreviation is used more than the full company name). You should never use abbreviations unless they are generally accepted e.g. EPS, GP, ROCE are fine, less standard abbreviations are definitely not.

Even if you produce a glossary of abbreviations be aware that they can interrupt the reading flow as the marker has to break off to look them up. The aim of a marker in both exams and the RAP is to do the job effectively and efficiently in the minimum time – anything that involves the marker in having to spend unnecessary time is not going to endear you and your work to them! Excessive use of non-standard abbreviations in extreme cases has been known to lead to a fail for communication.