Just like you do on your host, the kernel image, the initramfs if any, the kernel sources, the kernel modules, those ones are in /lib/modules/kernelversion, and kernel config, because many ebuild would like to check it for options. So not all are need, but you will get an easier life having them all copied.
The easier way (less pain and copy) should be build kernel and modules, copy kernel source tree (with modules and image built in it), and make install on all hosts.
Once copied, remember to select that new kernel as default on each hosts (see eselect kernel list).