Since you plan on keeping it a dim tank you want to stay with low light plants. Anubias
, most Cryptocoryne, and most Echinodorus. As for a floating plant there are lots of options there to two of my favorites are Duckweed
and pennywort the pennywort is actually a stem plant that can be planted or floated it looks nice either way. Of the Cryptocryne my favorite are the cryptocryne wenditt because it comes in three colors red, bronze, and green which give a nice contrast in the tank. Of the Echinodorus I love the Amazon Sword
because it gets big, the Ozelot Sword
because I love the green leaves with the red spots, and the Echinodorus Angustifolia "Vesuvius" because I love its twisted leaves. I have kept all of these, except the Echinodorus Angustifolia "Vesuvius" which I have ordered but not recieved yet, they have been very, very easy to grow. Also alot of the stem plants are very easy and they grow fast and can be trimmed to make more plants which I really like to.
As for the gravel the Eco-Complete is really good but I resently set up a soil base tank and it has amazed me the growth my plants have had in just the 2 week or so its been set up. But if you go with regular gravel it will work fine to just make sure you get small smooth gravel its better for the plants. I would go with a dark or natural color as it will make your fish look nice I don't think I would do black because it may make it hard to see you beautiful BGK.
Edit: Just wanted to add it will be cheaper for the plants if you get them online most of us on here order from sweetaquatics.com and aquariumplants.com.
You will need a liquid fertalizer I would recommend Seachem Flourish and if you go with normal gravel and get swords I would also suggest Seachem Root tabs because the sword will need the extra mierals and vitamins for there roots.
Also like Sincrisis stated if you put lots and lots of plants you will not have to worry about the cycle as the plants in a way will kind of automatically cycle the tank. For example when I set up my 55 gallon I put lots and lots of plants and that same day I put 7 golden barbs, 6 Neon Tetra
, 2 honey sunset gourami, 2 peacock gudgeon, and 6 albino corys in the tank and I did not have any problem at all with ammonia, nitrites or nitrate and my fish were and still are in great health.
Sorry the post is long and please feel free to ask any questions we are here to help.