I'd like to grant a specific gcloud user (a member) a role (a right) to start/stop a specific gcloud instance through gcloud console? Or do I have to make some scripting envelope to do so as the only option?
As far as I understand and according to the documentation:
If your project is enabled for Google Compute Engine, then the team members get access to all Google Compute Engine resources in that project as described by their roles, for example: viewer, writer or owner. If team members have edit permission, then they can modify instances and also access the instances using ssh.
So if that particular user has edit permission, that user would also be able to start/stop the other instances in the same project.
If you just want to give a user the ability to connect to a virtual machine instance as an SSH user, but don't want to grant them the ability to manage your Compute Engine resources, add a user's public key to the project, or add a user's public key to a specific instance.
So unless you only have that particular instance in a separate project, I don't think there is an option in the IAM & Admin section to do that.