Great question — answering as the UphillTech team:
You can use UTM parameters in the URLs you enter into your GMB listing. https://support.google.com/analytics/answer/1033863 https://ga-dev-tools.web.app/campaign-url-builder/ The values you enter will override GA's source/medium and campaign values. I set the source/medium to google/organic to indicate Google Search and set the campaign to something that identifies the listing. e.g. "GMB-12345". You can then identify GMB visits via the campaign.
Full answer by Tony McCreath: https://webmasters.stackexchange.com/a/138931 (CC BY-SA 4.0).
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