It can actually be a positive thing to complain, the aim of which is to make an improvement.
It could be that the old GP is clinically negligent.
As awful a situation as it was for you, the fact that your old GP could not reach a diagnosis and treatment plan is unfortunate, but not unusual.
Doctors are humans (on this site they appear to be merely someone to complain about and abuse, not that your post is in that vain by the way) and as such make mistakes, also people are not cars and medicine is not simple.
The training and experience of two doctors will never be the same and it is a FACT that however clinically excellent your new GP may or may not be he/she will have patients that they have difficulty or in fact never manage to diagnose and treat.
If two doctors saw the same 10 patients it is possible that each doctor would have a different way of managing them.
However that said there are some doctors that are in fact clinically negligent, but the mis-diagnosis of one patient does not place a GP in this category.