How to specify 'os' for free gitlab runners ('runs-on' equivalent)

How can I specify the operating system in the .gitlab-ci file to indicate which underlying OS on which the gitlab-runner should be running when using the 2,000 minutes of free execution in with gitlab.com shared runners in Google Cloud Platform?

For context, Github's free account supports 2,000 free minutes per month in "Github-hosted runners" for running Github Actions in Azure and MacStadium. You can specify whether you want the build to happen in Linux, Windows, or MacOS via the runs-on key in the github workflow yaml file.

What is the equivalent method for specifying that I want to run a CI/CD job in a specific OS on GCP in the .gitlab-ci file?

You can specify that you want gitlab to execute your CI job on a windows machine using tags.

Gitlab made Windows Shared Runners available as Beta in Jan 2020. Their announcement shows how to use the tags key in your .gitlab-ci file to specify Windows machines.
You can also run in a docker container in windows:

• https://docs.gitlab.com/runner/executors/docker.html#using-windows-containers

For example, I was able to execute a simple "hello world" powershell script with the following job in my .gitlab-ci file:
windows:
script: "build/windows/buildExe.ps1"
tags:
- shared-windows
- windows
- windows-1809


And here's the buildExe.ps1 powershell script reference above:

Write-Host "listing contents of C drive root"
Get-ChildItem -Path C:\ -Force

Write-Host "listing contents of cwd"
Get-ChildItem -Force

The above config produced the following output in my CI job:

```
[0KRunning with gitlab-runner 12.9.0 (4c96e5ad)
[0;m[0K on windows-shared-runners-manager 6QgxEPvR
[0;msection_start:1590842482:prepare_executor
[0K[0K[36;1mPreparing the "custom" executor[0;m
[0;m[0KUsing Custom executor with driver autoscaler dev (64a348d)...
[0;mCreating virtual machine for the job...
Virtual machine created!
section_end:1590842624:prepare_executor
[0Ksection_start:1590842624:prepare_script
[0K[0K[36;1mPreparing environment[0;m
[0;mRunning on PACKER-5E557E8E via
runner-6qgxepvr-wsrm-87cf700a3349de62a18e...
section_end:1590842640:prepare_script
[0Ksection_start:1590842640:get_sources
[0K[0K[36;1mGetting source from Git repository[0;m
[0;m[32;1mFetching changes with git depth set to 50...[0;m
Initialized empty Git repository in C:/GitLab-Runner/builds/maltfield/cross-platform-python-gui/.git/
[32;1mCreated fresh repository.[0;m
[32;1mChecking out 91f0d9e3 as master...[0;m
From https://gitlab.com/maltfield/cross-platform-python-gui
* [new ref] refs/pipelines/151176370 -> refs/pipelines/151176370
* [new branch] master -> origin/master
git-lfs/2.8.0 (GitHub; windows amd64; go 1.12.2; git 30af66bb)

[32;1mUpdating/initializing submodules...[0;m
git-lfs/2.8.0 (GitHub; windows amd64; go 1.12.2; git 30af66bb)
section_end:1590842661:get_sources
[0Ksection_start:1590842661:restore_cache
[0K[0K[36;1mRestoring cache[0;m
[0;msection_end:1590842670:restore_cache
[0Ksection_start:1590842670:download_artifacts
[0K[0K[36;1mDownloading artifacts[0;m
[0;msection_end:1590842677:download_artifacts
[0Ksection_start:1590842677:build_script
[0K[0K[36;1mRunning before_script and script[0;m
[0;m[32;1m$ build/windows/buildExe.ps1[0;m
listing contents of C drive root

Directory: C:\

Mode LastWriteTime Length Name
---- ------------- ------ ----
d--hs- 12/11/2019 2:47 AM $RECYCLE.BIN
d--hs- 2/25/2020 8:52 PM Boot
d--hsl 12/11/2019 10:29 AM Documents and Settings
d----- 2/25/2020 8:16 PM Git
d----- 5/30/2020 12:44 PM GitLab-Runner
d----- 2/25/2020 8:16 PM GitLFS
d----- 2/25/2020 8:34 PM Go
d----- 9/15/2018 7:12 AM PerfLogs
d-r--- 2/25/2020 8:35 PM Program Files
d----- 2/25/2020 8:27 PM Program Files (x86)
d--h-- 2/25/2020 8:51 PM ProgramData
d--hs- 2/25/2020 8:10 PM Recovery
d--hs- 12/11/2019 10:28 AM System Volume Information
d----- 2/25/2020 8:32 PM tools
d-r--- 5/30/2020 12:43 PM Users
d----- 2/25/2020 8:38 PM vcpkg
d----- 2/25/2020 8:51 PM Windows
-arhs- 2/25/2020 8:45 PM 408834 bootmgr

StackExchange DevOps Q#11723, answer score: 1