IIGS replacement capacitor

It should be marked with the capacitance value and the voltage.  Post the markings or a close up picture of the side of the capacitor with the markings on it and someone can translate it for you.  Once you know those numbers you can find what capacitor to buy.  I'd probably replace all 5 of those yellow ones and the two big black ones on the left side of your pic just to be on the safe side.  Maybe even the two blue ones on the bottom right.  But the main ones you probably want to replace would be the big gray ones marked .22 uF to the left of the large black electrolytics.  Those look like the type that often blow in Apple power supplies that are very stinky and messy when they do.

While youre replacing yes for sure - all 5 of the the yellows, the 2 blacks, and the 2 .22mFd silver 'safety' caps on the left.  Caps are cheap and easy to replace.  Just pay attneion to the + and - sides and make sure the replacements are the same microfarad and the same or higher voltage.  Those yellow and black canister 'radial' lead type capacitors are all on their way to death, and the silver safety caps although not electrolytic and not apparently RIFA brand might as well be replaced just in case.  I wouldnt necessarily worry about the blue foil capos - they are essentially orange drops but in blue and won't often fail, but if they come in your cap kit mifght as well get those too.  Check out this site https://console5.com/store/cap-kits/computer-cap-kits/apple-2-series.html   One of those GS kits is probably yours and they are cheap you the whole set.  If not just hit the contact us thing and email the guy...his name is Egan and he can help you assemble a whole kit for cheap.  Those yellow and black electrolytics dry up and then die or leak, and the silver safety caps seem to always age and explode at some point. A cheap fix to last another 40 years.  

That capacitor is a 1000uF 10v.