pinyin: gǔ

Chinese phrase: 古代 gǔ dài - ancient times
古怪 gǔ guài - strange, odd

Examples:
故
pinyin: gù;
Chinese phrase: 故事 gù shì
- story
故宫 gù gōng - the Imperial Palace, the Forbidden City

估
pinyin: gū;
Chinese phrase: 估计 gū jì - estimate, appraise

姑
pinyin: gū;
Chinese phrase: 姑娘 gū niáng - girl
姑姑 gū gu - father's sister, aunt