No problem for me if you want to port it, as long as the support to users is given by you. Have in mind that I don't support devices I don't own. If a company wants my support, the first thing they need to do is sending me a sample.
There are some aspects you need to consider in the porting though. You'll have to use stock kernels from the original devices, like iPEGA or JXD. G7 kernel in other devices won't work just as it is. iPEGA and JXD only have Jellybean kernels (not sure if JXD released something with kitkat), so the rom you need to port is the Legacy based on JB. The reason is that bootloader in JB and KK are different in Rockchip, and Kiktat adds a metadata partition that doesn't exist in JB (Selinux too), so you cannot use kitkat kernels in Jellybean.
Also, if stock kernel from other devices like JXD or iPEGA has some issues, like a bad refresh rate, it will be difficult to fix that without proper source code from these devices, so it would be a LegacyROM with a buggy stock kernel.
Also, stock mapper by GPD won't work in other devices, because joystick driver will be different, specially the way they are connected internally to the mainboard.
So, to sum up, basically until JXD or iPEGA release an official kitkat stuff, the ROM you can port is the Jellybean one.
Anyhow, if I can assist you somehow, let me know, though I cannot test anything because I don't have any JXD device or the iPEGA one.
Oh another thing I almost forget, apart from kernel, you should need to modify parameter file included in RK devices to determine the size of partitions (I don't know the nand memory in other devices).
About the full SDK, GPD can hate me if I distribute them. Though you can contact them and probably they will release it to you. The kernel changes are in my github though (although kernel is quite unusable if you don't have kernel source code for JXD or iPEGA), or at least a defconfig file for those devices