。
I like playing the guitar.
Literal
I [topic-は] guitar [object-を] play [nominalizer-の-が] like.
Classic nominalization: ギターを弾く is a verb phrase, の turns it into a noun-like unit, that nominalized clause becomes the grammatical subject of 好き with が. This is the standard way to say 'I like [doing X].' Compare with the alternative ことが好き — の and こと both nominalize, with subtle nuance differences.