We have a suite of UI tests (Functional) that use GEB. However the code is legacy, not following CSS componentization and one change in CSS can have undetected side-effects in unrelated areas. Is there any plugin or workflow that can be integrated with GEB to provide Automating CSS Regression Testing?
In the JS world there exists BackStopJS for Javascript. Is there anything analogous for GEB?
You could simply take a screenshot of every meaningful page or page state that you want to verify, and compare those between test runs.
Call browser.report("some page") in Geb, and it will dump a screenshot.
This would be the easiest way to detect any changes, CSS-related or otherwise, in the rendered pages, using the setup you already have.
In case there are many pages with changing images giving you false positives, you might want to use placeholders.
Also, instead of comparing the screenshots directly, you could calculate the difference between the images, and / or exclude regions from the comparison.