There's no alternative.
ごご is the correct reading for 午後.
Unfortunately, that message doesn't mean you answered ごご.
In Kanji practice mode (and in Tests, too), you just can see "Answer Correct" or "Answer Wrong" AND the right answer.
You never see what option you've chosen, unless you choose the right one.
In Vocabulary Practice you can see your mistakes, but not in Kanji Practice.
I'd also like to always see what answer I've chosen when I fail.
I'm sure this change was already suggested some time ago. If not, please, beeant, add this to the wish list.