In Matthew 5:5 why does it say that the gentle (meek) will inherit the
earth? Is it talking about the new heaven and the “new earth” when Jesus
returns or is it something else?

“The meek shall inherit the earth” is a truism. A truism is a
general statement which can be applied to a broad range of situations. It
is similar to a proverb. Another example of a truism in the Bible is “A
man reaps what he sows.” (Galatians 6:7) This statement of Paul’s is
very similar to Proverbs 11:18 as well as 2 Corinthians 9:6. “A man reaps
what he sows” has a wide variety of applications, as does “The meek will
inherit the earth.” It is a true biblical principle that from God’s
perspective and even in practical ways in this world, the meek will
inherit the earth.

Having said that, it is clear that the most obvious
application of the truism uttered by Jesus in Matthew 5, and probably the
one Jesus had in mind when he said it, is that the meek will inherit the
“new heaven and the new earth,” as you say. In fact, Jesus may even have
had Zephaniah 3:12 in mind when he made the statement in Matthew 5. “But
I will leave within you the meek and humble, who trust in the name of the
LORD .” In the prophecy in Zephaniah God is definitely talking about
heaven.

So I would say that it is true that Matthew 5:5 is implying
that those who are humble and meek will inherit the new earth of
Revelation 21:1, but that it has many other applications. God will bless
and multiply the work and ministry of all those who are meek and humble of
heart, both in this life and in the life to come. As the psalmist says;
“But the humble shall inherit the land, and shall delight themselves in
the abundance of peace.”

John Oakes