Error «Error response received from Steam. Please try again later. If the problem persists, contact Steam Support: Error 1000.50» in Social club during GTA 5 startup can be caused by not properly work of the client or Steam servers. Error 1000.50 can be corrected in most cases by itself, without any additional actions.
UPD 21.10.2015: Error 1000.50 directly related to the Steam servers work. Servers simply cannot cope with a large number of people willing to play GTA Online. The only solution to this problem, it is only waiting for correction by Steam.