This wood is grey since the default brick color on Roblox is this exact shade of grey and it hasn't been assigned a wood type. It was first created by an exploiter (who will remain unnamed for the sake of privacy). This exploiter used a program which would fill blueprints with a specific filename, and when no filename was assigned, Grey Wood was created (since Roblox's default block color is grey). There used to be extremely few blueprints. However, due to rapid duplication in the community, it became very common.
Grey Wood can rarely be found on some player’s bases. On January 5th, Defaultio released a patch stating that he had fixed the grey wood exploit. However, around May 10th, 2017, a new glitch was founded to spawn grey wood, therefore making the patch obsolete.