Like the other Science Weapons in The Outer World, the Gloop Gun does not really do a lot of damage, and there are many better guns out there if you just want to kill enemies as quickly as possible. However, this is a fun gun to use, and can be pretty useful for certain situations.
So, how do you get the Gloop Gun in The Outer Worlds? Like the other science weapons, it's hidden in a container somewhere on one of the planets (or moons). This particular Science Weapon is found in an area called UDL Lab on Monarch, not too far away from Stellar Bay. You will naturally get to this area when you are doing the quest called "Errors Unseen", but you can go there any time you want.
When are are in the UDL Lab, you will need to either hack a computer, or if you don't have enough hacking skills, answer three questions to open the door. The answers are as follows:
1) "Protect the Chairman".
2) "Berate him relentlessly to defend the Chairman's honor".
3) "Arrest your spouse and admit your children to a reeducation program".
This opens up the container where you can loot the Gloop Gun. Now just take it, and it's your to keep forever. Congrats with your new Science weapon in The Outer Worlds.