GLOO Cannon is one of the unique weapons in Prey. Its basic uses involve slowing down or stopping enemies in place with the glue the cannon shoots. It is especially useful while fighting Mimics and other agile aliens. The cannon won't kill any enemy but will let you use other weapons easier thanks to weakening the opponent.
The cannon can also be useful during exploration. You can create places that can be helpful in climbing, glue any leakages in the station or get rid of electric discharges or falmes.
The cannon can be found in the final part of the march through the Neuromod Division. It lies on the ground in of the main corridors (the above image) and is very hard to miss. You'll know the place when January informs the protagonist about the possibility of grabbing the first neuromod (the Everything you know is about to Change side quest).