No, Naruto and Gaara aren't best friends. But the important part is that they have a deep friendship built from recognizing themselves in each other and they always support each other.
Naruto is one of the first people who truly understands Gaara's loneliness because he's lived through a similar kind of isolation. Their relationship has that feeling of, "You understand a part of me that almost nobody else can."
And their connection becomes lifelong. So even though they aren't constantly together, they're important to each other's lives and remain connected long after their initial encounters, cheering one another on from the sidelines in everything without envy, even when the other surpasses them in something monumental.
When Naruto learned Gaara became the Kazekage, Naruto wasn't jealous, he was delighted and it only further pumped him up to catch up. When Gaara saw that Naruto undoubtedly surpassed him in power, he didn't hold that against Naruto. It only fueled his admiration and reverence for Naruto's spirit.
If Gaara had been Naruto's teammate instead, Naruto would've loved Gaara above everyone else. (Dude just latches onto his teammates like that π )
