Yes, it's pronounced longer, however, it is not called "gachec" but "čárka" - I am speaking about this little comma written over a letter: " ´ " so you get vowels like á, é, í, ó, ú, ů, ý, all of which are pronounced longer (á like aa).
By gachec you probably meant háček " ˇ " which can also be written over come consonants or ě so you get letters like ě,š,č,ř,ž,ň,ď,ť - their pronunciation is different from the letters without háček over them. E.g. ň is like Spanish ñ, š is like English sh, č is like English ch in the word check, etc.
AJ_1112's question may have stemmed from an inconsistency in how that is taught in other languages in Duolingo. I see that learner is taking Spanish (at least had some by the time I saw this post. In Duo Spanish from English course, "you all" is used widely in lessons to distinguish when it's the ustedes vs tú version of the Spanish verb needed for the correct answer. It is a very controversial issue in that course, and I am certainly not saying it should be dealt that way here too. Just giving some possible context to the OP's confusion.