I have a question regarding predictor variables in a GLMM analysis. 

I am running a study investigating multi-modal communication in primates,
specifically, primate gestures that accompany vocalizations. 

I have measured the call rate, call duration and peak frequency (response variables) for 200 calls (100 accompanied by gestures, 100 without gestures). 

I would like to use the following predictor variables: 1) Call type, 2) Presence of gesture, 3) Gesture type. 

Is it possible to use GESTURE TYPE here as a predictor variable? I am not sure if it's appropriate since this will only be for 50% of the overall the data (where calls are accompanied by gestures).