Completely different concepts. For one, Akainu's meteors are the result of him launching magma from his fists into the air, and then them falling down. Those can be embedded with Haki because they are, at one point, in contact with Akainu's body. Fujitora's meteors, on the other hand, aren't in contact with Fujitora's body so he can't imbue Haki into it as it's falling. The fact that Sabo, when hit directly with his gravity wave after Fujitora started fighting seriously, simply dispersed into flames suggests that Fujitora can't imbue Haki into his gravity. If he could, he most likely would have done so, which would have resulted in Sabo being damaged by the gravity, rather than just turning into fire.
