"Port 47" is not the same as "Protocol 47". See /etc/protocols for a list of all of the strange, weird, odd, and haven't-been-used-in-decades-by-anyone IP protocols. You are already familiar with TCP (protocol 6) and UDP (protocol 17), which are the only two protocols that use ports.
Your need a pass rule for "proto gre" rather than a redirection rule for port 47.
However, AFAIK since no port is involved no redirection rule will apply. I suppose you might route the packets, instead, with route-to.
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