At the present moment, I am not aware about any GA GCP "native" services, which can be used directly from Google. From my point of view, there might be a few workarounds to consider.
1/ External third party service, which can be used as a storage, so you can fetch from it, or can be configured to forward to your GCP storage bucket.
2/ Market Place solutions. I think there quite a few of them there. WIth different functionality and cost.
3/ Deploy and maintain a proper COTS SFTP server (might have storage buckets as a backend).
4/ Convince your partner to use their own SFTP server, and fetch data files from it. In that case, there might be plenty of options how to do that - from self developed serverless solutions (my SO example is described here: serverless solution for external SFTP access), to GCP "native" solutions (you mentioned in your question), and to third party solutions.
5/ some variation/combination of the above.
I would guess that your decision might be based on the many non functional requirements, and heavily/ultimately influenced by direct and indirect costs (COTS software, GCP expenses, third party partners, people - might be the main driever, etc.).