Neither of your lenses can be fitted with a truly functional lens collar.
And, to work properly, a gimbal should be paired up with a large lens that has a lens collar.
In other words, it's not "gimbal vs. lens collar"... It's gimbal AND lens collar. Your lenses have no means of fitting a collar AND aren't really large, long or heavy enough to handle properly on a gimbal. So neither a gimbal nor a lens collar is a solution to your problem.
To solve your problem, the easiest and best solution would simply be a better tripod head that holds better and doesn't slip.
If your tripod head uses Arca-Swiss type quick releases, it also might be possible to fit your camera with a long mounting plate that's aligned with the lens axis, rather than tangential to it the way camera plates are normally fitted. This would allow you to slide the camera backward slightly, to improve balance and equilibrium, reducing the tendency of the tripod head to drop.
Or, if you already have an Arca-Swiss camera plate on the bottom of your camera, get a Wimberley M-8 Perpendicular Bracket that fits onto the camera plate but protrudes forward, and will allow the same sliding adjustment for better balance on the tripod head.
Yet another possibility is a lens support. Kirk Photo offers a number of different ones (
https://www.kirkphoto.com/catalogsearch/result/?cat=&q=lens+support) but they are lens-specific and none is made for Nikkor 28-300 or 105 Micro. You might find a more universal bracket of this type, but it may interfere with some functions of the lens, such as the manual focus or zoom rings. To be helpful, it too much be fitted with an Arca-compatible lens plate and allow for the whole rig to be slid backward atop the tripod for better balance.
You also might try using smaller, shorter prime lenses like 50mm f/1.4 and 85mm f/1.8 or f/2. These also have larger aperture than either the 28-300 or 105mm, so may be able to more strongly blur down backgrounds.
Personally I rarely use a tripod when I'm shooting portraits.
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Neither of your lenses can be fitted with a truly ... (