לְשַׁבָּחָא עַל כִּנָרָא דְאָתְיָא מִן גַת עַל יְדָא דְאָסָף.
1. For the conductor | לַמְנַצֵּחַ – See Rashi and Ibn Ezra on Tehillim 4:1 that the verb "לנצח" means to supervise, as per its usage in Ezra 3:8 and Divrei HaYamim II 2:1. As such, the term "מְנַצֵּחַ" might refer to a musical director. Cf. R. Saadia (cited by Ibn Ezra there) that the word is connected to the noun "נצח", eternity, and that the title implies that the psalm was established to be sung or played perpetually.
2. the Gittit | הַגִּתִּית – The term appears only three times, all as parts of headings to various psalms. See Rashi and the commentary attributed to Rashbam that it refers to a musical instrument made in Gat, or Ibn Ezra (Tehillim 8:1) that the psalm was given to the descendants of Oved Edom the Gittite to sing. Cf. Radak there that it refers to a certain known melody.
3. Of Asaf | לְאָסָף – According to Bavli Bava Batra 14b, Asaf composed the psalm. Cf. Ibn Ezra in his introduction to the book that Asaf might have simply been in charge of singing it.