Docker should automatically request permission to have access to the Windows filesystem. In the case that this doesn't happen, it is easy to remedy by right-clicking on the Docker status icon, selecting "settings" and then under "Shared Drives" ensure that the C drive is checked (or wherever the Users directory is), and click "Apply." After Docker is running again, restart the Gigantum Client.
Otherwise, when Gigantum Client won't start or gets hung on Windows (either using Gigantum Desktop or Gigantum CLI) it is probably because something went wrong between Docker and Windows. This is particularly frequent when the system goes to a "deep" (i.e., low-power) sleep state.
The simplest general solution is to stop Gigantum Client, restart Docker, and when Docker is running restart Gigantum Client.