Every character on Jiǎoluò Shūwū read as gu in any tone — 2 readings total across the four tones. Pick the tone you mean.
If you have the sound gu but not the character, here are the 2 characters read this way: 古 (gǔ, ancient, old, classical), 故 (gù, reason; old; deliberately; therefore). They split across 2 tones, so the tone is what separates them.