What does (G+) (G++) in terms of male mutation rate and male-heterogametic XY-system?
I think you need to provide more context.  When I saw G++, my first thought was the C++ compiler of that name.  I am unaware of any universal use for the term in biology.  It could very well be context specific.  It is common to use that sort of notation when describing alleles in a population, with the pluses following whatever variable name you gave to the allele.  So if we had an allele that we called G, and an individual had two copies of that allele, it would be common to see that individual referred to as G++, with G+ being used to refer to someone with only one copy.  I think it is quite possible that this is what the term means in your context but it is impossible to say for sure without more information on what the question you are being asked is in reference to.  Maybe it would help if you posted questions 1-4 or the blurb that precedes the problem set so we could see what you are actually being asked.