EGAN Paper With Confusing Notation
$\nabla_D$ means the gradient w.r.t. the discriminator parameters. The authors explained below as "the log gradient value of updating D".

BTW it's not standard to write something like $\nabla-a-b$ as this paper, which confuses readers as "$\nabla$ minus $a$ minus $b$". It's much better to write $-\nabla(a+b)$.