After adding static ARP entries 'arp -s 172.16.1.5 <MAC>' to the ARP tables on each end HostA and HostB respectively, ping (and tcpbench) work fine end to end through the gif tunnels, from both sides.
Thus my problem appears to be limited to ARP Requests resp. their Ethernet broadcasts not being forwarded properly by the bridges if they come into the bridge via an EtherIP tunnel from a gif interface...
