By
publicly posting or displaying the content you give other end users of
the Services a perpetual, sublicensable, irrevocable, worldwide,
royalty-free, and non-exclusive license to reproduce, adapt, modify,
translate, publish, publicly perform, publicly display and distribute
(subject to the restrictions set forth in Sections 11.4 and 20.3 of
these Terms) any Content or derivative works thereof which you publicly
post or display on or through the Services.
According to 11.1 (a) and 11.3 (a), modelers retain
all
copyright except that Google and other users are allowed to re-use and re-distribute the content, unless they explicitely put their
work
under a different license (but most modelers don't do this).
That
means if a modeler doesn't mention a different license in the model
description, it is under Google's royalty-free license of 11.1 (c). Unfortunately most modelers miss to mention any different license, but get angry if someone nevertheless uses their models.
Best is always to contact the owner
to ask about permission, especially if you want to use the works in a
commercial project. Most modelers have enabled a setting which shows an
envelope icon next to the author's name on the model page.
I hope that helps ;-)