Why does it say "Gam"?
Radak: Just like they will do good, Hashem will give His good, i.e. rain.
Malbim: Initially the Mizmor described the beginning of the sprouting and of the reflection. It compares Emes to seed that begins to sprout from the ground. The Tzedek opposite the Emes is like rain 'reflected' from Shamayim; it is prepared to descend when Emes will sprout from the ground.
How will Hashem give Tov?
Rashi: He will open His storehouse - Shamayim, to give rain, so "our land will give its yield." Radak - "Yiftach Hashem Lecha Es Otzaro ha'Tov Es ha'Shamayim Lases Metar Artzecha b'Ito" (Devarim 28:12).
Why will our land give its yield?
Radak: It will not be lacking any good.
