AC6 + ST-Link V2 + SWD programming
This thread comes up when searching for this problem and the solution isn't quite correct so I'm replying even though it's old.
It makes no sense to have the line reset_config srst_nogate by itself. This was probably discovered by randomly editing the config files until it worked. The srst_nogate option would only make sense if you are actually using the nSRST line, and removing srst_only means that nSRST is not being used. The reason this works is only because of the default behavior of the reset_config option.
The correct setting for programming a board that does not have nSRST connected (i.e. software-only reset) is:
reset_config none