Can we use GAEs with A3C asynchronous reinforcement learning?
Sure can:

https://github.com/ikostrikov/pytorch-a3c

The reason is the GAE paper was released after A3C, so the paper implementation, which is what most people will look at, doesn't use GAE.

Relevant lines here: https://github.com/ikostrikov/pytorch-a3c/blob/master/train.py#L95-L98