This question was answered by @SphericalCow:

according to http://linux-ip.net/gl/ip-tunnels/node4.html, there should be support from kernel versions 2.2 and probably above. Debian 8 seems to be fairly recent release, so the chances are that it is supported. I haven’t used the Debian kernels with DO, so I can’t say for certain.

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Hi,

I’m late but this could help others so here is my answer.

IPIP/GRE are kernel modules so all you need is to check the kernel module in /lib/modules/$(uname -r)/kernel/net. They should be both under ./ipv4 if supported. enable them by modproble ipipormodprobe gre