The account posted a copy of the letter Trump's lawyers wrote to Rep. Jamie Raskin (D-Md.), the House of Representatives' lead impeachment manager.
Requests for comment to Trump's political action committee and his lead impeachment defense lawyer weren't immediately returned.
Former Trump campaign lawyer Jenna Ellis joined Gab this week. Other current and former advisers are active on the website.
Trump had not posted there since Jan. 8.
On that day, he posted twice, including a notice that he wouldn't be attending the inauguration on Jan. 20. Trump left Washington on Inauguration Day, hours before his rival Joe Biden was sworn into office.
Trump has floated starting a rival to the companies that banned him. Parler, an emerging competitor to Twitter before being kicked off Amazon's servers, never drew the former president, but he has been a member of Gab since August 2016. Trump has over 1.3 million followers on Gab, which owns its own servers after facing a deplatforming effort several years ago.
"Most technology startups have the luxury of using third-party cloud hosting providers like Amazon AWS, Microsoft Azure, and others. Gab does not have this luxury. Over the past four years we have been banned from multiple cloud hosting providers and were told that if we didn’t like it we should 'build our own.' So, that’s exactly what we did," CEO Andrew Torba wrote in a blog post last year.
Gab bills itself as "The Free Speech Social Network" and says its mission is "to defend, protect, and preserve free speech online for all people." Gab emphasized on its website that "political speech that is protected by the First Amendment of the U.S. Constitution will be allowed on the platform." Illegal activity, such as pornography and threats of violence, aren't allowed.