The GD-ROM is at present the only optical disc format used by Sega Dreamcast and the arcade system it runs on, Naomi. Stopping their production may very well mean a stoppage of future game production as it would imply the following:
No more NAOMI arcade games. No more NAOMI ports to Dreamcast (Recent NAOMI ports include Under Defeat and Radirgy), Trigger Heart Exelica (February 2007) and Karous (March 2007) will unexpectedly be the final official Dreamcast games, the NAOMI 2 will no longer be supported, neither will the Chihiro arcade board, GD-ROM capabilities of the Triforce arcade board will no longer be supported.
Even Sega has recently presented Dynamite Deka EX running on NAOMI, so it doesn't make a lot of sense but maybe they know what they are doing.
Nobody seems to find the reason why Sega is stopping the production of GD-ROM, so they who first announced it on a site also made it possible for others to state their opinions on the matter, advising to show their opposition.
However, even if not a lot of statements have been made, it is certain that Sega of Japan plans to discontinue production of GD-ROM media in February, 2007. Time will reveal more.