It all depends on how you use Gaia. If you use Gaia to make a website then pick apart the created code and figure out how it did the things it did then it will help you learn immensely.
On the flip side if you just publish websites using it and don't bother to pick it apart to learn the internals then it will simply be a crutch allowing you to never learn how to do it yourself. An ability to create something that you could not have otherwise created, and if you cannot use it again in the future, you will not have any clue of how to do it.
Use it as a learning tool, not just a building tool.
EDIT: That was all general comment on building tools in general. Specifically relating to Gaia I'd like to add that personally I think most top end devs make their websites MUCH MUCH better than the Gaia framework does. As a matter of personal opinion I'd only recommend using it as a base learning tool, then building your own framework that you can constantly improve on as you get better.