。
She kissed him gently.
Literal
She [topic-は] him [to-に] gently kiss [object-を] did.
優しく is the adverbial form of 優しい (i-adjective: 'kind, gentle, tender'). Modifies the kiss as an adverb of manner. キス is one of the cleanest English loanwords in Japanese — phonetically straightforward, fully naturalized. キスをする ('do/give a kiss') is the periphrastic verb form; キスする also exists as the suru-verb. に here marks the recipient of the kiss.