Does a creature that gains haste after it enters the battlefield still have summoning sickness that turn?

Summoning sickness is governed by CR 302.6, which states that a creature cannot attack or use activated abilities with the tap symbol unless it has been under its controller's control since the beginning of that player's most recent turn — unless it has haste (CR 702.10a).

The key is that haste is checked at the moment you want to attack or activate a tap ability (during the declare attackers step or when you want to activate the ability). If the creature has haste at that moment, it is exempt from the summoning sickness restriction, regardless of when it gained haste. There is no lingering "sickness" that haste must cure before a certain window — it simply bypasses the restriction while it is present (CR 509.1b for attacking, CR 302.6).

If the creature loses haste before you act, summoning sickness applies again at that moment. Haste must be present at the time you are trying to attack or tap.

Example: You play Grizzly Bears (a vanilla 2/2 with no haste) on your turn. On the same turn, you cast Expedite targeting it, which gives it haste until end of turn. You can now immediately tap Grizzly Bears to attack, because it has haste at the moment attackers are declared.

Bottom line: gaining haste mid-turn fully removes the summoning sickness restriction for as long as haste is present. The creature does not need to wait an additional turn.