I have exact same situation currently. There is a post on these forums where someone created post for Fusion 360 and Slant Pro to export mixed Turret/Gang tooling code. I have not tried it since I am little concerned about how different it is from standard post... When I have time I will get into the code to check.
In Fusion 360 it can generate negative values for gang tooling but you have to specify the in Post-Process screen useGangTooling=YES. This will generate the negative X values and spin the spindle in reverse. Problem is that you cannot mix Turret and gang tooling in single post so you have to stitch things by hand and then if you forget and post code for Turret with gang tooling set to yes unpleasant things will happen...
I decided to simply get left hand tooling to use as gang tools so they approach from top and not need negative X values or reverse spindle spin... It makes programming easier and I don't have to stitch things manually.