The granules in gel caps end up having air gaps making them float. You can crush the granules to pack them more, but I fill the gaps with powdered red clay and a bit of dry micros, which gets them to sink.
Wet gel caps are slippery AF! The difficulty of putting them in discouraged me from using all the ones I made for a long long time. Till I made this:
The easiest way to use osmocote is to put some down before adding substrate. But after that it gets to be a bit of a chore.
I've used the Flourishroot tabs as well and those are little dense blocks (clay with nutrients baked in?), which ensures they don't float and are really easy to use. If you buy an off the shelf root tab, look for videos and photos of what they look like cuz some of these commercial products are little more than gel capped osmocote as well. I think they go with the opaque gel cap so you don't see that it's just osmocote inside lol.