How do you save custom fields on an EntryFixture?
Prepending the field name with field: should do the trick.
&lt;?php

return [
    [
        'sectionId' =&gt; $this-&gt;sectionIds['locations'],
        'typeId' =&gt; $this-&gt;typeIds['locations']['locations'],
        'title' =&gt; 'Location 1',
        'field:gmbId' =&gt; '123',
    ],
];

See https://github.com/craftcms/cms/blob/develop/tests/fixtures/data/entry-with-fields.php for reference.