Sorce code uses literary copywrite. If the publisher is out of business, then the rights revert back to the author. G5 is the author of TvT. If Lighthouse did not use TvT to settle some portion of it's debt and did not sell it, then you need permission from G5.
As to abandomware (quote Wikipedia)
In most cases, software classed as abandonware is not in the public domain, as it has never had its original copyright revoked and some company or individual still owns exclusive rights. Therefore, downloading such software is usually considered copyright infringement, though in practice copyright holders rarely enforce their abandonware copyrights.