In my experience, lugging a gimbal through the woods is a recipe for a sore back. I once tripped because of all that weight... it wasnt fun. For hiking, keep it simple:

• Sirui P-326 Carbon Fiber Monopod is light and supports the Sigma 150-600mm f/5-6.3 DG OS HSM Contemporary perfectly.
• Get extra Canon LP-E6NH Lithium-Ion Batteries. Safety first, you gotta stay mobile when bears are around.

To add to the point above: a gimbal might be overkill for hiking. These are decent budget alternatives for the Smokies:

• SanDisk 128GB Extreme PRO SDXC UHS-I V30 cards handle the 90D burst rate well.
• Lowepro Flipside 400 AW III fits that huge Sigma lens attached to the body.
• LensCoat Lens Cover for Sigma 150-600mm Contemporary helps protect the barrel. Let me know if you need more gear tips!

Oh man, you are gonna have such an amazing time in the Smokies! The 90D is a total beast for wildlife and that reach with the 600mm is gonna be fantastic for bear shots. Regarding the gimbal vs monopod debate, honestly it comes down to your stamina and how much you hate gear fatigue. Gimbals are incredible for tracking birds in flight or steadying long video clips because they basically make the setup feel weightless once it is balanced, but they are bulky as heck. Before you drop the cash tho, I gotta ask: are you planning on doing mostly roadside shooting from the car or are you actually trekking miles into the backcountry? Also, are you more of a handheld shooter who just needs a break sometimes, or do you prefer having the camera locked down and ready to go for long periods?

Ugh, literally same. I've been having the worst luck trying to get my 90D wildlife kit sorted lately and it's honestly been such a letdown. Every time I think I've got it figured out, something else feels off or unreliable... it's exhausting tbh. I'm actually stuck on the same gimbal debate too so I'm no help, but you should definitely just search 90D wildlife setup on YouTube or check the dedicated 90D subreddit. There's this one guy who made a massive guide that covers every single accessory you'd ever need for the Smokies. It's much better than anything I could explain right now while I'm still struggling with my own gear issues lol.

Building on the earlier suggestion, it seems the group is largely favoring the monopod for trail work, which makes total sense for your Smokies trip. Having used both setups for years, I am very satisfied with the monopod choice for long hikes. Just a few things to watch out for as you finish your kit:

• Watch out for generic battery brands. While Canon ones are pricey, some cheaper alternatives can actually throttle the 90D frame rate, which is a total disaster when you need that burst for wildlife shots.
• Be careful with how you balance that Sigma lens. Compared to native Canon lenses, the Sigma contemporary glass is quite front-heavy, so a cheap ball head will likely slip and could potentially damage your gear.
• Skip the budget shoulder straps. Carrying that 150-600mm on a thin strap will cause significant fatigue by noon. Honestly, sticking with a solid monopod instead of a bulky gimbal will keep you much more agile. You really dont want to be faffing with a massive gimbal mount while a bear is walking into a clearing.