Greater restoration does not give special knowledge of the effect it cures.
Spells only do what they say they do.
The caster must choose which of the five effects of greater restoration to use (PHB, pg. 246):
You imbue a creature you touch with positive energy to undo a debilitating effect. You can reduce the target’s exhaustion level by one, or end one of the following effects on the target:
- One effect that charmed or petrified the target
- One curse, including the target’s attunement to a cursed magic item
- Any reduction to one of the target’s ability scores
- One effect reducing the target’s hit point maximum
They can choose to end a charmed effect, but they learn nothing about how the charm effect came about, since the spell doesn’t say anything about learning anything about the source of the cured effect.