Can you attack with a creature the same turn it gains haste from an aura?

Normally, a creature with summoning sickness — one that has not been under your control since the beginning of your most recent turn — cannot attack or use tap abilities (CR 302.6). Haste overrides this restriction entirely (CR 702.10a).

The key timing rule is CR 509.1: you declare attackers during the declare attackers step of combat. As long as the creature has haste at that moment, summoning sickness does not prevent it from being declared as an attacker, regardless of how it gained haste — whether printed on the card, granted by a static ability of an aura, or any other source.

If you attach an aura that grants haste during your main phase before combat (e.g., by casting it), the creature will have haste by the time the declare attackers step arrives and can attack freely. If somehow the aura falls off before that step, the creature would lose haste and could not attack (assuming it would otherwise have summoning sickness).

Concrete example: You play a Grizzly Bears on your turn (it has summoning sickness). You then cast Ghitu Firebreathing (or any haste-granting aura) on it during your pre-combat main phase. When combat begins and you reach the declare attackers step, Grizzly Bears now has haste, so you may attack with it that same turn despite it just entering the battlefield.

In summary: yes, an aura granting haste is just as valid as any other haste source — what matters is that the creature has haste at the moment attackers are declared (CR 509.1, 702.10a).