For GPU testing you need GPU infrastructure, which is not available in freely accessible GH runners/CI. You can either self-host GPU runners, or use the JuliaGPU Buildkite runners (somewhat separate CI from GH actions). You can see in packages like KernelAbstractions.jl how both CPU and GPU CI is handled (via .github and .buildkite files).
One alternative could be, if your package supports both CPU and GPU backend, to for now only have CPU testing in CI, while running GPU tests locally…