Hydrochloric acid which is sold as brick cleaner will eat all of the rust, you'll soon find out if you've got any "pepper holes"! The stuff I have is min. 36% acid and is very strong, goggles and rubber gloves are a must as is doing the job outside. Probably what you can buy at a "public" DIY store will be weaker, it will still do the job but take longer. I have enough to fill a tank and have cleaned several with it, always over a big plastic container as a couple have developed leaks as the acid's done its job.
The acid won't touch "gunge" this will need washing out first, while we're on the subject of dodgy substances............. A strong solution of hot Caustic soda shifts most gunge, followed by a few very good rinses as it's an alkali and needs to be totally removed before the acid goes in.
A wash out with washing soda (alkali but much weaker than caustic) will clean out and neutralise the tank afterwards, I then splash some paraffin around the inside to protect it before fuel goes in. Once the rust is removed you can clean it further for a sealer if needed.
These are some before and after pics. of a Bantam barrel, I seem to remember this was after about a 2 hour soak in the acid.
'53 Bantam, '55 Huntmaster, '61 Bantam, '79 GS 850, 2004 Burgman 650, 2002 Burgman 400