Automatic merge from submit-queue (batch tested with PRs 57351, 55654). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
GCE: Get automatically created subnetwork if none is specified for auto network
Fixes#57350
**Release note**:
```release-note
GCE: Fixes ILB creation on automatic networks with manually created subnetworks.
```
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