In terms of bike weight the full suspension will be the heaviest (and most expensive) and the gravel bike will be the lightest (and often have drop handlebars). The hardtail is in between and well suited to gravel and dirt trails. The full suspension is better suited for trails that are twisty and have incline sections as the rear tire can provide better traction.
It is purely subjective but I own a hardtail (with dropper post) and a full suspension mountain bike. I tried out a Specialized gravel bike but for riding on the pavement it lacked the feel of a regular road bike and the dropper seat post rattled quite a bit. If I lived in an area with miles of gravel or dirt roads or trails that were mostly level as with the bike paths on former railbeds, it would be different and a gravel bike would be a good choice.
If you have areas where you plan to ride it would be worthwhile to go to the parking area and talk to the bicyclists and learn why they chose the bike they are riding.