I am getting 1986 Chevy Chevette and it has been sitting in a old ladys garage for 20 years and only has 27,000 miles on it. Will i need to replace all the gaskets and rubber as well as the battery?

Maybe not. But first thing is to remove all the bad gas out of the gas tank and put a new fuel filter on it. Remove fuel line at engine and allow fuel pump to pump new gas into the line. Drain cooling system and replace the water pump. Change the spark plugs,(might even spray a little wd 40 or marvel mystery oil in each cylinder) , Change the oil and filter. Now if it cranks and idles(only let it idle don't rev up) I would add some sea foam to the oil(as directed on the can) crank it back up and let it idle for thirty -forty five minutes(only idle) this will help clean all the old oil and crude out and should free up the hydraulic lifters(if it has them). Change oil and filter again. Be sure cooling system is operating properly and it doesn't overheat while doing the above. Inspect brakes. Might even go ahead and replace the pads, calipers and rubber brake lines going to the calipers. Now if you get this far and all is looking ok and it's running ok. Take it for a spin. You will probably notice a blue smoke coming out the tail pipe as you take off or right after each time you start it up. This will most likely be the valve guide seals. They not all that hard to replace. Replace them and the valve cover gaskets and you should be good to go for now. But keep a close eye and ear for leaks and funny sounds. After a day or two of driving I would consider changing the transmission fluid and filter(if auto trans). The above is what I did with a 1985 z28 that had set for 15 plus years. All is good with it and no major issues since. Hope this helps.