I have a similar configuration, however i defined in site B the default gateway GW A.
My idea is that all trafic in site B destinated to internet will flow throght the VPN and exit to the intervnet via GW A.
With this configuration PCs on site B can open only some internet websites. If i do a speedtest (speedtest.com) i can only measure download speed, upload speed test fail all the time.
Is this firewall problem?