The generation and amplification of terahertz (THz) electromagnetic waves by plasmonic instabilities in conventional two-dimensional (2D) electron systems (2DESs) have been actively investigated since 1980 [1]. However, after about forty years, we are still a long way from the realization of efficient emitters and amplifiers [2]. The rise of graphene and its extremely strong light-plasmon coupling and superior carrier transport properties make this work worth to be revisited [3]. We investigate dc current driven plasmonic instabilities in high mobility graphene-channel field-effect transistors (GFETs) working for tunable THz amplifier at room temperature (RT).