I'm new to using Linode, but tried them out because they had a build for Gentoo. Of course when I installed it, I realised that it wasn't configured to use Systemd, and now I'm left wondering how to set that up given that the typical migration path (https://wiki.gentoo.org/wiki/Systemd) expects things like Grub to be installed.

Is it generally safe to just:

# emerge gentoo-sources
# zcat /proc/config.gz /usr/src/linux/.config
# make oldconfig
[ make my required changes to the startup arguments ]
# make && make modules_install
[ copy the new kernel to... where? ]

...and then restart, or am I asking for pain? Is there a default Gentoo image that I can start with that uses systemd? What's the typical usage here?

It turns out that this was about as easy as I thought. I did the above and coupled it with the kernel changes required here: https://wiki.gentoo.org/wiki/Systemd, then followed this Linode guide: https://www.linode.com/docs/tools-reference/custom-kernels-distros/custom-compiled-kernel-with-pvgrub-debian-7-ubuntu to get the kernel working with their system.

The only tricky bit was networking, since that's different under Systemd. For that, I simply put this in /etc/systemd/network/50-static.network:

$ cat /etc/systemd/network/50-static.network 
[Match]
Name=eth0

[Network]
DHCP=yes

In retrospect, the name "50-static.network" doesn't make much sense since I'm using DHCP, but it doesn't really matter I suppose.