Everyone's in favour of the free market until it does something they disagree with. Anyone can start a new browser. Gab could fork Chromium or Firefox, even. Or they could probably just make a bookmarklet. But that wouldn't get any attention and hate groups like Gab thrive on controversies like this. HN is doing them a favour here. They're not doing anything new or original (remember the Genius annotator, anyone?).
In fact gab is participating in the free market exactly how you'd expect a believer in the free market to act, they're creating their own browser: https://github.com/gab-ai-inc/defiant-browser