The ad boycott is believed to have taken a significant toll on finances at GB News, which last year saw its losses balloon to more than £42m.
The start-up broadcaster has since been forced to cut jobs while seeking new sources of revenue from live events and a paid membership scheme.
However, it remains deeply reliant on billionaire investor Sir Paul Marshall and Dubai-based hedge fund Legatum, who jointly own GB News through holding company All Perspectives.
The amount GB News owed to its parent company almost doubled from £42.8m to £83.8m in the last year alone.
Mr Musk’s legal action highlights a major escalation of tensions between the advertising industry and media outlets that have placed an emphasis on free speech.
Linda Yaccarino, chief executive of X, said the actions of the Global Alliance for Responsible Media (Garm), whose members include consumer goods giants Unilever and Mars, had cost the business billions of dollars.
X and Rumble are seeking unspecified damages for allegedly violating US anti-trust law.
The lawsuit, filed in Texas, also names the World Federation of Advertisers (FWA), which launched the Garm initiative, as a defendant, alongside UK advertising behemoth WPP.