I noted that some games were removed from the database, gran turismo 2 (U) disc 2, star wars dark forces (U), etc.. I was curious what is the reason of this removal?

weren't they red or grey dumps?

They were dumped by [unknown] dumpers, I was asked to hide these dumps until redumps are made.

Someone has stickied the relevant thread for Gran Turismo 2 in the dumps area.