Note that DREAMPlace is a global placer; it also comes with a detail placer but global placement is what it is targeted at. I don't know of an appropriate research analogue for the routing phase of the problem that follows placing, but maybe someone else does.
For example, the VTR people are looking at RL to pick what actions to take during FPGA placement for their simulated annealing placer (see: https://ieeexplore.ieee.org/document/9415585).
I also know this paper/poster was indexed recently under OpenRevew but I'm still waiting for some more implementation details to look at: https://openreview.net/forum?id=6GR8KqWCWf
FPGA routing I don't think anyone has touched on using ML/DL but I do know that there is some talk about using ML/DL models with current routing approaches to replace search heuristics (think like replacing or augmenting something like A*) or do routability predictions. Certainly, there are probably many ways to use RL in routing as there are many places in current algorithms to intelligently make certain heuristic decisions.
Edit: I also want to note that there are a ton of works that also use ML/DL to tune the "hyperparameters" of EDA tools such as placers and routers. Think ML/DL for back-box non-linear optimization.
Do you mean it uses gradient descent to find the optimum?