since you mentioned Helm v2, I did not want to point you directly to the latest head. Instead, the Git history hides the source of the Helm chart’s default configmap:
Looking at it, and digging deeper into the repository, I can see the values.yaml you can override.
I’m curious though where
runners.kubernetes.volumes.host_path is coming from - this is not provided by the Helm chart, only discussed in MRs which are not merged (Add ability to define kubernetes volumes (!39) · Merge requests · GitLab.org / charts / GitLab Runner · GitLab). Are you maybe using a custom Docker image to deploy in the Helm chart for the runner?
Digging a bit deeper, the generation of the runner config.toml from the values.yaml seemed limiting and was addressed in Allow user to specify any runner configuration in the runner helm chart (#106) · Issues · GitLab.org / charts / GitLab Runner · GitLab and Allow user to specify any runner configuraton (!271) · Merge requests · GitLab.org / charts / GitLab Runner · GitLab - though this was 13.6 in Nov 2020, whereas 0.16.0 of the chart was released in April 2020.
That being said, chart versions 0.23.0+ allow to specify a custom config template for the runner. 0.27.0 adds a configmap override. More advanced configuration is described in the docs.
In your release, you are limited to the static value mapping between values.yaml and configmap.yaml defined by the Helm chart itself. There are workarounds discussed in the linked MRs with a custom Docker image for deployments. I would not go there though but instead upgrade the chart, and test the different versions and configuration options.