Measure the width of the gate you plan to install, plus the outside frame of the gate, including the clearance space for the gate latch and hinges. For a simple gate, you'll cut the wall from top to bottom.
Mark the first vertical line on the side where the gate will be hinged. Line the cut up with the ends and centers of a vertical column of blocks so you will have a solid face for the hinge side. Snap a chalk line along the outside edges of the ends of the blocks so you have the full thickness of the block for strength on the inside face of the opening.
Measure the gate opening width at the top and bottom from the first line and mark the wall. Stretch the chalk line between the marks at the top and bottom and snap the second line. Go to the other side of the wall, measure and mark two vertical chalk lines in the same place opposite the first two marks. You will have to cut from both sides because the saw won't cut deep enough to go all the way through.
Attach a fresh carbide or diamond-impregnated blade to a portable concrete saw. Be sure if you are doing this yourself that you have the upper body strength to handle the weight of the saw. Get someone to help you who has experience with these types of saws if you don't. Attach a fresh carbide or diamond-impregnated saw blade to your circular saw.
Put on protective glasses and a dust mask. Score the chalk lines with the circular saw blade to provide a straight, inch-deep groove for the concrete saw blade to follow. Begin near the top and set the base of the concrete saw against the wall. Press down against the base and make a plunging cut into the scored groove. Cut from the initial cut upward and cut to the top. Drop down a foot or two and make another plunging cut and work up the groove upward to meet the bottom of the first cut. Keep making short cuts working toward the bottom.
Cut the second line on the same side. Go around and cut the lines on the back side of the wall. As you cut the last line, use the sledge hammer to break loose upper blocks and remove them. This reduces weight on the wall below. Removing blocks you've cut free will help prevent the freed wall section from overbalancing and falling on you, or breaking and damaging the wall as you work on the lower cuts.
Use a mallet and chisel to clean up any edges that are ragged or broken. Use a grinder with an abrasive disk designed for concrete to smooth and clean up the edges. Pour concrete into the blocks nearest the cut to strengthen the wall on either side of the gate. This makes a sturdier mount for the gate. Use a hand trowel and mortar mix to fill in any gaps along the edges of the cut to make a smooth edge around the gate opening.