Protecting 3/16″ wire rope going through poles

Depending on the weight of the gate and your climate gate post holes are dug deep enough so as to not be effected by frozen ground heave. If you are located in this type of climate than 48 inches should be your goal to avoid having the posts disturbed by frozen ground forces. If you reside in a mild climate than 18-24 inches will suffice. The diameter depends on the diameter of the post, but figure for a 3-4 inch post drilling a 5-6 inch round hole. To help keep the post from possibly loosening the last 3-4 inches of the hole bottom should be widened to form an undercut (or a bell shape) making it wider at the bottom. A gas powered auger will save much time and labor. You will still need a hand operated post digger to remove the last of the dirt. Be forewarned hitting a rock will be a jolting experience even with a clutch. Once you have excavated a post hole to the proper depth insert the post. It should be positioned (if you drilled exactly) approximately in the center. You should support the post so that it remains in position while the concrete sets or you make adjustments. Any length of lumber or rebar that is available will do. Attach two opposing braces close to the top of the post (wire will fasten to anything well). Using an appropriate sized level, check the post for plumb alternating from 90 to 135 degrees on the pipe circumference (side to side and front to back). Adjust the support brackets to keep the post plumb. You should have at least (6) 60 lbs. bags (or equivalent) on site to fill a 48 inch deep x 6 inch diameter hole. Mix two bags at a time in a wheelbarrow following the directions for the amount of water (I believe it's 2 gallons per 60 lb. bag). When completely mixed shovel into the hole. After every 4-5 shovels tamp the concrete with a 2 x 2 or 2 x 4 to compact the mix and remove the air. The job will be most successful if you mix and pour the concrete without letting it set up. Fill the hole so that the concrete is a few inches above grade. With a trowel round off the concrete so it tapers away from the post. Finally, don't forget to inscribe the date and your initials in the still wet concrete. You can remove the post supports the next day, but let the concrete set a few days before hanging the gate.

The chain link fence folks have all kinds of fence post caps that could be used to keep the rain out. Also there are nylon caps for EMT. You could convert the tubing to iron pipe threads and install threaded PVC caps. As to the holes and lacing the cable, before installing poles, I would put them on the drill press and pre-drill.