Gathimba emerged at international level in 2014, taking a silver medal at the 2014 African Championships in Athletics behind Lebogang Shange of South Africa. He set a personal best of 1:23:59 hours to place second at the national championships that year, behind David Kimutai Rotich. He topped the podium at the 2015 national trials with a new best of 1:23:12 hours, which ranked him in the world's top 100 for the first time. He again demonstrated his place among the continent's best at the 2015 African Games where he lost out on the gold to Shange by a margin of one second.