估 (gū) is a bound morpheme — it never appears alone as a standalone verb in modern Chinese. You must combine it with another character in compounds such as 估计 (gūjì) 'to estimate', 评估 (pínggū) 'to evaluate', 估算 (gūsuàn) 'to roughly calculate', and 估价 (gūjià) 'to appraise'. Learning these common words is more useful than trying to use 估 by itself.