How fine is the gravel? If it's too coarse then it can damage chickens feet and cause bumblefoot (as can woodchips and the like). If it's fine or has a fair bit of sand in it then it's less likely to be an issue. They will enjoy scratching around and bathing in it, even if the soil quality is poor. The chickens will usually improve the soil condition over time anyway.
Depending on the area of the concrete and the gravel, it's up to you whether you bother with a topping over the concrete. If they will have plenty of area to be in un-concreted, then you don't necessarily have to put anything down on the concrete. If overall the area is small then maybe a thick layer of sand wouldn't hurt over the concrete. You could try some fast growing screening plants in pots around the concreted area, it would help reduce the amount of heat that is reflected and provide some shade too.
You could plant out a bunch of pest repellent plants in the planter boxes, some are bushy and will provide shade, however check which are safe for poultry. And you'll probably want to restrict access because chickens can decimate plants. You could also have a crop of fodder for the chickens to forage in, you can get seed mixes or make up your own. If you screen them off with mesh set above the plants the chickens can nip the tops off without totally destroying the plants, enabling continuous growth.
Shadecloth can make a huge difference to heat levels, particularly a light colour cloth. I've got a 50% white shadecloth (nominal 30%) in my garden, and you can really tell how much cooler it is underneath. If you could get a light coloured 70-90% cloth it would make more of a difference too.
You could try installing a high pressure misting system if you really wanted to. The superfine misting heads are designed to instantly evaporate the water once it's in the air, which can have a great impact on cooling areas without making things wet. Some people have set up thermostat controlled ones that switch on when it gets over a certain temp and stay on until the ambient temp drops. Fairly easy to do with Arduino nowadays even for beginners. But you can manage temps without any of that.
Anyway, I'm rambling, good luck with your pending chicken adventures