Don't use detergent (some say a very weal bleach mix is OK but I've never even risked that).
I'd just wash the gravel with tap water and then scald with boiling water.
Should be enough.
Wipe out the tank with hot clean cloth or scrub with a clean nailbrush, or whatever you like really.
Diseases will usually not survive the process, or at least they haven't on me.

Then you either cycle the tank from scratch or get some dirty filter media from someone here.

I use plain old lemons i would rub the tank down with the lemon, place a bowl or something that will hold boiling water with the pieces of lemon in it, cover the tank an hour or two later i rinse and wash the tank again allow to dry when dry wait one day then

sorry about the fish.

just noticing that you changed his tank; did you do anything to seed the filter in the new tank with bacteria ? if not, and you didn't e.g. use the same substrate, you basically had an uncycled tank.

as regards starting again, if you still have one cycled tank (the smaller one), you can do a proper clean of the bigger one, and just use the filter in the smaller one (in fact run a spare, the one you're going to use in the bigger tank) and hey presto instant (practically) cycle...

The tank that I moved the goldfish into was my old tropical tank. It had been running for about 18months then it was empty for about a month before I moved the goldfish in. During this time I ran the filter from it in my new tropical tank (Juwel Vision 180) so it shouldn't have lost it's cycle.
Substrate was the same but had been left dry for some time.
I treated all water with conditioner and let the water heat to 19C before I moved the fish, I even brought a good portion of the water from the old tank.
No I have three tanks, the old goldfish tank which now has a male Betta in it, the tank the goldfish died in which has been empty since the fish died and will now be uncycled and my tropical tank.
At a loss now as of what to do with the middle tank.