This study investigates whether there is a consistent category of free functional auxiliaries for tense, aspect and modality (TAM) marking or agreement in Greek Sign Language (GSL). Such markers are described but also compared with other signed and spoken languages, as well as with cross-linguistic tendencies. The study demonstrates that GSL has signs that attach to the verb, similar to auxiliaries in spoken Greek or English. Characteristics of auxiliaries such as systematic use, use across all groups of verbs, load of semantic content, as they are illustrated in classic studies on cross-linguistic tendencies provide the necessary framework. In a supplementary manner, findings from wide samples of languages test the initial claim of a closed category AUX and imply that there is a broader, open-ended category of functional TAM and agreement markers, some members of which are finally ‘drawn'' into a closed class of AUX, often just to move further into grammaticising as free or bound markers., covering different stages in this continuum of grammaticisation. Possible indications of a Creole status and gestural roots in GSL are also examined.