Shouldn't it be "I really like you."?
That keeps the same meaning, but duolingo may not accept once the 1st translation for really in realmente.
So "Gosto realmente de ti" sounds right to you? Would anyone actually say this?
Yeah... as well as "gosto demais de ti"
isnt that i like you too much
When would people say that? Too much means excessive, a negative.
Demais is not always negative in Portuguese...
Oh, might be getting into deeper cultural issues there.
How are você and ti used? I assume that ti can only be an object in a sentence, but você can be a subject or an object. Is this correct?
For example, would 'gosto muito de você' still be correct?
yes, you can use você here. It's even more usual!
What is the difference between te and ti? And is there a difference in the pronounciation?
The pronunciation is the same =) TI is used after prepositions. (De ti, for instance, but not the "com ti", but "contigo" - 'with you').