Can a creature attack the turn it enters the battlefield if it gains haste after summoning sickness already applied?

Summoning sickness is not a one-time debuff that permanently marks a creature — it is a continuous condition checked at the moment you declare attackers. Specifically, CR 302.6 states that a creature cannot attack unless it has been under your control since the beginning of your most recent turn, or it has haste.

Haste (CR 702.10a) simply removes the restriction from CR 302.6. The game checks whether the creature currently has haste at the time you declare it as an attacker during your Declare Attackers step (CR 508.1a). There is no persistent 'sickness counter' or memory of past states — only the current game state matters.

So if a creature enters the battlefield without haste and later gains haste (e.g., via an activated ability, an aura, or an equipment) before you reach your Declare Attackers step, it is fully eligible to attack that same turn.

Concrete example: You cast a Grizzly Bears (a vanilla 2/2) on your main phase. Before combat, you activate the ability of Fires of Yavimaya granting it haste until end of turn. When you reach Declare Attackers, Grizzly Bears currently has haste, so it may attack — even though it entered the battlefield that very turn without haste.

Conversely, if the creature loses haste before attackers are declared, it cannot attack. The check is always made at declaration time, not retroactively.