GBIF does not store iNat place boundaries. If you want to use iNat boundaries, you can download the kml (e.g. https://www.inaturalist.org/places/geometry/greater-austin.kml), convert to geojson (which can be done with an online converter), and upload the boundary to GBIF by pasting the text of the geojson into the appropriate box.
In this case, GBIF also had to simplify the boundary afterwards, but the shape is mostly preserved.
Perhaps the bigger issue, as textileorchard mentioned, is that GBIF doesn’t take all iNat records. It takes all Research Grade records with either CC0, CC BY, or CC BY-NC licenses. If that matters to you, you should use iNat’s export tool instead.