Is it permissible to sell gold that a mother gave to her daughter, without the mother's knowledge, in order to spend it on those in need in Syria, knowing that the mother might be angry about this action?

If your mother gifted you the gold, then you are not required to seek her permission to dispose of it by selling or giving it as charity. However, if doing so would anger her, then you should not do it, for showing kindness to parents is among the best deeds.