Stretching the Metrics tab in Inspect Element's Right side pane of Elements tab will display the css box model with padding, border, margin and position.
If you are using Safari version 9.1, you can see it by selecting "Styles - Computed" in the Styles dropown in the Styles drawer on the right of the dev tools. (The Styles section otherwise defaults to "Styles - Visual", so you need to make the selection to make the box visible). Hope this helps.
The question was about grid inspector. There is no grid inspectors in Safari. Although Safari is noted for Safari Responsive Design mode there is no grid inspector in Safari yet.
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