How do I get rid of gaps in words when I justify paragraphs in mac word?

It is important to edit with non-printing formatting marks displayed.

Does the last line end with a paragraph mark or a manual line break?

The latter will spread words across the line.

Word does really bad justification, relying solely on inter-word spacing for the effect. In Word 2010 and 2011, there was a compatibility setting Do full justification like WordPerfect 6.x for Windows that did a much better job, because it used letter spacing as well. Unfortunately, that option has been removed from all current versions of Word, both Mac and Windows.

I recommend you avoid justification in Word, left-aligned text has been shown to be more readable.

Yes! That's it. Here's a macro to set that:

Sub ApplyWPJustify()

    ActiveDocument.Compatibility(wdWPJustification) = True

End Sub

To make every new document have this setting, you can open Normal.dotm in Word, run the macro and resave. Or you can change the macro name to AutoNew macro and add it to Normal.dotm, so every new blank document has the setting.