"Morning. You're up early today." "You're one to talk — I figured you'd still be snoring away."
Literal
Morning. Today [topic-は] early [ね]. / You [emphatic-こそ], still snore-snore was-sleeping [quotative-と] was-thinking [despite-のに].
こそ on a pronoun throws the emphasis back at the listener — 'you, precisely' — like English 'you're the one to talk.' ぐうすか is a mimetic for deep snoring sleep, a variant of ぐうぐう. 寝てる is the contracted ~てる for 寝ている. 思ってた is 思っていた contracted, past progressive. のに trails off — 'I was thinking X, but [you're not].' Writing アナタ in katakana adds a teasing/informal affect the kanji/hiragana forms wouldn't.