There is no subway to Gubei Carrefour or City Supermarket in Gubei. I have seen busses in the area and know that they go there, but do not have any idea of the bus schedule. If you take Line 1 subway to Xu Jia Hui, then you are about a RMB12-14 ride away from Carrefour. Note that the quantity of imported goods at Carrefour isn't as great as City Supermaket or The Pines Marketplace, but the prices are less.
There is a Carrefour that is located near a subway. Take line 1 to Lian Hua Road (I think that is it, but I haven't been out that way in a while... it is the stop after or two stops after Jingling Park) and there will be a large Carrefour and some other shops. This one does not stock as many imported items as some of the others do.
If you want to go to another City Supermarket that does have subway access, you can go to the one on Huai Hai Road via Metro Line 1, exiting at Shanxi Road. The supermarket is in the basement of Primtemps. I'm sorry that I don't recall which exit number that is. Alternatively, you can take Line 2 to either Shimen Er Road or Jinan Temple and then walk to Shanghai Centre/Portman for a small City Supermarket Branch. I wouldn't recommend going to the Times Square branch that can be accessed by taking Line 1 metro to Huang Pi Nan Road.
Whatever you do will be insignificant, but it is very important that you do it. - Mahatma Gandhi