Nothing really too it. Take fish out and put them in a tub. Keep your filter moist. Drain tank. Remove rocks by hand, See if you can clean tank with no cleaners just water. Put new rocks in. Fill tank. Use Dechlor. Bring up to temperature. Put filter back and put your fish back.
NOW a couple more important things. You should have a good liquid based test kit. You need to test the water every 12 hours for ammonia and nitrite. Anything over 0 for either and you need to do water changes. Gravel changes can cause mini-cycles in tanks. This just means that you will have disposed of some good bacteria with your gravel and the existing colonies will have to catch up. Takes a week or two. Second, you have some mighty big fish. I don't know about the "catfish" as there are 100s but I do know that a 30 gallon is too small for kissing gouramis. If all these fish are big, you need to be particularly careful with the water quality and the potential for mini-cycles.
Good luck.