Laser light generation is intimately related to processes that generate single photons. To date, gravitational waves have not been detected, and there are no known processes that produce single gravitons (not to mention there is no direct evidence that the gravitational field is quantized at all -- just logical arguments based on the structure of general relativity and quantum mechanics extrapolated to the relevant regime).

Since there aren't any processes known to produce single gravitons, there is no known means by which one could produce a gravitational wave laser.

Laser light generation is intimately related to processes that generate single photons. To date, gravitational waves have not been detected, and there are no known processes that produce single gravitons (not to mention there is no direct evidence that the gravitational field is quantized at all -- just logical arguments based on the structure of general relativity and quantum mechanics extrapolated to the relevant regime).

Since there aren't any processes known to produce single gravitons, there is no known means by which one could produce a gravitational wave laser.

EDIT: I agree with anna v's answer and John Rennie's comment, and I hadn't thought about free electron lasers when I wrote this. It would take relativistic planets or something like that, but it wouldn't be impossible.