Gimbals are definitely not just selfie sticks. They're for producing cinematic quality video and are quite effective when used correctly. I've got a DJI Osmo Mobile that i was using with my S7 but I havent had a chance to try out the S8+ on it yet. I'm going to guess that it'll be pushing the limits on the motors for it though.
Anyway... just set it up for the S8+... Looks like it'll work alright but no way your gonna get it to work inside a case and you've basically got to adjust the thing to the limits of the device. I'm sure the weight and size will put stress on the motors and use more battery than normal, but it does work.
Sorry for the sideways pic:
Of course...this won't help you with the GoPro part