noun or participle which takes 'suru', adverb
be disappointed; feel let down; lose heart
The main modern use. Common as がっかりする for feeling disappointed or discouraged, and also adverbially as がっかりと to describe a dejected manner.
試験に落ちて、彼はがっかりしていた。
He was disappointed after failing the exam.
When she heard the result, her shoulders sank in disappointment.
noun or participle which takes 'suru', adverb
feel exhausted; feel drained
Much less common than the disappointment sense; it refers to losing energy or feeling worn out. In ordinary modern Japanese, ぐったりする or 疲れ切る is usually clearer for physical exhaustion.
「長旅でがっかりした」のがっかりは、疲れ切った意味で使われている。
The gakkari in “I was worn out from the long trip” is being used to mean exhausted.