Almost done, all that's left is to add a barrel to our turret. Anywhere in the top view, make a block that is 64x8. Move the new block so that it's left side is flush with the center right side of the turret top. That might have confused you a tad, so look at this image to see how it should look so far. Well, now the actual construction of the turret itself is done, but there's still some stuff to add.
Every turret needs an origin brush.
It tells Half-Life what point the entity is rotating around. An origin brush is just a normal block with the 'origin' texture covering every side, much like a clip brush. Open the texture browser and type 'origin' in the filter. Double-click on any 'origin' texture that shows up. Now, make a new brush in the top view, anywhere you want.