Nearly always read gū: 估计 (to estimate), 评估 (to evaluate), 低估 (to underestimate). A second reading gù survives only in the old word 估衣 for cheap secondhand clothes, rarely heard today.
亻 on the left is the two-stroke side form of 人. Estimating is a judgement made by a person, so the radical names the appraiser. It pairs naturally with 价 price in 估价 (to appraise).
古 on the right supplies the sound, gǔ shifting to gū with a tone change. The same phonetic powers a tight family: 姑 aunt, 故 reason, 苦 bitter, 居 to dwell.
