Because Эта гитара. Это не гитара.
Эта гитара = this guitare = article + noun. No verb. The article like in Latin languages (Italian, French, Spanish etc...) agrees in gender with the gender of it's name.
Этот as the article for masculine noun.
Эта as the article for feminine noun.
Этот стол. Masculine. Etot stol, this table.
Эта гитара. Feminine.
It's the "this" that is just before a noun, as its article.
This is = Это.
No masculine or feminine. It's the "this" that is just before a verb.