According to practically all sources, it was November 30, 1998. Which makes sense, since it did come out near the end of the year.

Yeah, it was in 1998. But does it matter?

The original Baldur's Gate came out in 1998, yes, but it didn't have Tales of the Sword Coast with it as that came out in 1999. As this is the Original Saga edition, which contained the TotSC expansion, the release date given by GOG isn't as inaccurate as you make out.

According to Wikipedia, Tales of the Sword Coast was released on April 30th 1999.

However, the retail Original Saga edition came out in 2002, but that was a re-release of the double pack anyway, so it just gets silly working out release dates for it.