Because Glary Utilities reinstalls it via an auto-updater. To stop it permanently, uninstall the entire Glary Utilities suite.
Gcinst.exe is an executable file that is part of the Microsoft .NET Framework. The ".NET Framework" is a software framework developed by Microsoft that provides a large library of pre-built functionality, known as the .NET Framework Class Library, and a virtual execution environment, known as the Common Language Runtime (CLR). The .NET Framework allows developers to create a wide range of applications, from Windows desktop applications to web applications and mobile apps. Gcinst.exe
Suspicious locations include %Temp% , C:\Windows , or C:\Users\[User]\AppData\Roaming\ . Because Glary Utilities reinstalls it via an auto-updater