Complex sentence: By December, Gibb and Barry announced that the Bee Gees broke-up, after that, Gibb recorded an unreleased solo album "The Loner" between 9 December 1969 and 25 March 1970.
Split into simpler sentences: By December, Gibb and Barry announced that the Bee Gees had broken-up. After that, Gibb recorded an unreleased solo album "The Loner" between 9 December 1969 and 25 March 1970.