Can you explain - what you mean - "EDA design tool for GaAs substrate"?
There are EDA design tools, for layout, schematic capture, LVS, DRC, parasitic extraction, circuit simulation, IR/EM analysis, etc. etc.
Most of them are commercial, but some of them - free.
What exactly are you interested in?
The place to start is the eventual foundry for this
GaAs "whatzit". They will be pleased to tell you the
design kits they offer and what platforms they are
built for. Anything that is not supported this way is
some combination of excess effort and increased
odds of "fail". With no support to be expected, since
you didn't get on the right bus and it's all on you.