Install Pari-gp

Create a simple script and store it as the file

simple.gp

for example.  Then in windows explorer right click on it and choose "Open with".  Then pick "Choose an app on your PC" and choose "C:\Program Files (x86)\Pari64-2-15-2\gp.exe" or wherever gp.exe has been saved.

Now clicking on simple.gp will load it automatically in gp.