you cant after 10.11, Apple removed it entirely so you can allow it on a per-incident basis only.
FWIW I do a lot of 3D work and use software that is developed by people who don't have any compulsion to register with Apple and do what they are told but provide amazingly great and free software because they are just totally cool people. GIMP is one of the products out on the internet that falls into this category - and I commonly see it suggested to game developers and amateurs who do not want to blow $$$$ on Adobe products which I support professionally and which have possibly the worst support staff in the world despite the tens of thousands of dollars we (the parent company I work with) pay them every year.
Gypsies had no home, the Doors had no bass player, GIMP developers do not have a developer license by papal decree from Pope of Apple. Do not let this frighten you, it should liberate you...
...Just download it from its souce page and nowhere else.
www.gimp.org
Apologies to "the Kids in the Hall".