grequests.post() function takes only one positional argument i.e URL to which you have send that POST request. The rest of the parameters are taken as key word arguments.
So you need to call grequests.post() as:
import grequests
urls = ['http://localhost/test', 'http://localhost/test']
params = {'a':'b', 'c':'d'}
rs = (grequests.post(u, data=params) for u in urls)
grequests.map(rs)