。
I slept like a log last night.
Literal
Last-night [topic-は] soundly sleep-potential-[past-た]-[sentence-final-よ].
ぐっすり is the state mimetic (擬態語) for deep, sound sleep — pair it with 眠る or 寝る and you've got a fixed collocation. The potential form 眠れた ('was able to sleep') captures the sense of successful restful sleep. The よ at the end adds a note of sharing good news.