includeIf directive supports as well conditions based on configured remote URL. [1]

  [includeIf "hasconfig:remote.*.url:https://client1domain.com/**"]
    path = ~/work/client1/.gitconfig
  [includeIf "hasconfig:remote.*.url:https://client2domain.com/**"]
    path = ~/work/client2/.gitconfig

Check comment [2] for a complete example with pinning SSH key and signing key using git-config.

[1] https://git-scm.com/docs/git-config#Documentation/git-config...

[2] https://news.ycombinator.com/item?id=36800853