If a creature is declared as an attacker and then gains flying before blockers are declared, can it only be blocked by creatures with flying or reach?

Flying grants an evasion ability that restricts which creatures can legally block it. Specifically, CR 702.9b states that a creature with flying can only be blocked by creatures that also have flying or reach. This restriction is checked when blockers are declared during the Declare Blockers Step (CR 509.1b).

The key timing here is that the game only checks a blocking creature's eligibility at the moment blockers are declared (CR 509.1b). If the attacking creature already has flying at that point — regardless of when it gained the ability — the flying restriction is fully in effect. How it gained flying (an aura, an instant, an activated ability) doesn't matter; what matters is that it has flying when legal blockers are determined.

It is also worth noting that if a creature somehow lost flying after blockers were already legally declared, the block would still be legal. Legality of blocks is only checked at declaration time, not continuously re-evaluated afterward (CR 509.1b, 509.2).

Concrete example: You attack with a Grizzly Bears (a vanilla 2/2). Before the Declare Blockers Step, you cast Leap (an instant that gives a creature flying until end of turn) on your Grizzly Bears. Your opponent can now only block the Bears with a creature that has flying or reach — a plain 3/3 ground creature cannot legally block it.