The copyright is held until it expires. If the code is explicitly GPL, then you can still use it - but you can't change the license or remove the copyright, and you have to abide by the clauses in the GPL version used. There are organizations that might sue to enforce the GPL license, even if the original copyright holders might not because they are defunct.
Note that someone owns that copyright, because someone owns the IPR of that company, even if it is defunct. It might be worth checking to see what company bought their intellectual property, or who the founders are (they may be the IPR holders too). It's a big longshot, but you might get permission to relicense the code under a different Open Source license, or a transfer of the copyright.