You can easily operate the Ikea devices on the Philips Hue Gateway. Not the other way round. The Ikea gateway is also pretty useless. With the Philips you have additional goodies regarding motion detectors. For example, push notification when you are not at home (via Apple HomeKit). Or that it should not disturb scenes.

hmmm good question, it works the other way round, but only halfway, because the IKEA motion detector has to be connected to the lamp to be addressed.
I recommend you stick to one system, why don't you just use the IKEA motion detector?