Dear Evon Margaret: I am not familiar with a gall that is found on Forsythia. There has been documentation on a root knot nematode (that forms galls on the roots), but to not aware of any stem forming galls. Galls are often times caused by insects feeding or laying eggs that later feed on the plant tissue formed by the gall and induced by the insect.
If you can, take a sample to your local extension office or a good retail nursery or garden center. They will be able to confirm the plant is a Forsythia and if not what the plant is for you. From there, identifying the gall and possible cure can be done. You may have discovered a new pest of Forsythia!
Good Luck!
