On 14 July 2014, Gassama signed a three-year contract with Real Valladolid, being assigned to the B-team also in the third division.[3] The following 26 January he terminated his contract and joined another reserve team, Rayo Vallecano B in the same category.[4]
Gassama was definitely promoted to Almería's main squad in Segunda División ahead of the 2018–19 campaign,[9] and made his professional debut on 17 August, starting in a 0–1 away loss against Cádiz CF.[10] His first professional goal came on 11 September, as he scored the equalizer in a 2–1 away defeat of Málaga CF for the season's Copa del Rey.[11]
On 1 November 2018, Gassama scored a late brace in a 3–3 home draw against Villarreal CF also for the national cup. The following 31 January, he moved to third division side Valencia CF Mestalla on loan until June.[12]
Gassama returned to the Rojiblancos for the 2019–20 season, being a first-choice under Pedro Emanuel and scoring a brace in his first match of the campaign, a 3–0 home defeat of Albacete Balompié.[13] However, he lost space after the arrival of Darwin Núñez and Juan Muñoz, being demoted to a backup option by new manager Guti.
On 28 January 2020, Gassama signed a four-and-a-half-year contract with La Liga side Real Valladolid, being immediately loaned to CF Fuenlabrada until June.[14] On 1 October, he was again loaned to Fuenla for the 2020–21 campaign,[15] but spent most of the season nursing foot and knee injuries.[16]
On 28 August 2021, Gassama moved to fellow second division side Málaga CF on loan for the 2021–22 season.[17] Roughly one year later, he moved to Racing de Santander in the same category, also on a one-year loan deal.[18]