Hmm, probably about 7kg of gravel? I can never remember. You'll need enough to cover the bottom and be about 3cm deep. If it's too deep then you get patches of anaerobic bacteria, if it's too shallow then plants can't root. If you were thinking of getting some little catfish then sand is a better choice than gravel, which can injure their barbels.
Decor depends on what fish you have and what your personal preference is. You could get bogwood (there are some plants that will grow on it that look really nice and are easy to grow), rocks (be careful what sort, some can alter the pH of the water), ornaments etc. Real plants or silk ones are fine, avoid plastic ones as some of them can have sharp edges. If you go for real ones then get something that stays fairly small. Crypts are a good bet. Real plants will need a planting substrate to go under the gravel.
There are lots of species that will live happily in a 60 litre. What is your tap water hardness and pH? If it's around neutral then you have loads of options but if it is very alkaline or acidic then that will limit your choices a bit more. Let me know once you've tested and I can recommend some fish.
I wouldn't go for anything that gets bigger than 2 inches (there are still exceptions) and I'd stick to just one shoal of one species or possibly 2 species if you were to have a shoal of catfish and a single specimen or pair of gouramis for example. But once you know your water type then I can start being more specific.
Hope that helps, happy browsing :)