Does a creature with summoning sickness lose that restriction if it gains haste after entering the battlefield?

Summoning sickness is not a counter or a persistent marker — it is a condition defined by whether a creature has been under your continuous control since the beginning of your most recent turn. The rule governing this is CR 302.6, which states that a creature cannot attack or use tap/untap abilities unless it has haste or has been under your control continuously since the start of your current turn.

Haste (CR 702.10) simply overrides that restriction. Crucially, the game checks whether a creature currently has haste at the moment the restriction would apply — such as when you declare attackers (CR 508.1a) or activate a tap ability. If the creature has haste at that moment, the summoning sickness restriction is lifted, regardless of when it gained haste.

This means that even if a creature enters the battlefield without haste and gains haste later in the same turn (for example, from a pump spell or a static ability), it can immediately attack or tap for abilities. There is no memory of it ever lacking haste — what matters is its current status.

Example: You play Grizzly Bears (a vanilla 2/2) on your turn. Normally it cannot attack this turn. Before combat, you cast Fires of Yavimaya's activated ability or a spell that gives it haste until end of turn. Grizzly Bears can now attack this turn, because it currently has haste when you declare attackers.

Conversely, if a creature loses haste after it was already declared as an attacker, it remains an attacker — removal of haste does not undo a legal attack declaration already made (CR 506.4).