The head I would make by laminating either two or three layers. If it’s to be painted it doesn’t matter, if it’s to be oiled / varnished I would always make it using three layers. With three layers you get the same joint appearance on both sides. The top layer should be two pieces butted at 12 o clock. The second or middle layer should be three pieces joined at 10:30 and 2:30. The third layer is the same as the top. Three layers also helps to ensure that you avoid any twist or bow (virtually) completely as the stack is balanced. If using slab sawn wood, make sure you orientate the growth rings opposing each other on the two layers or on the top and bottom layers in three ply. With three layers you can also ‘build’ in the mortise avoiding an operation. Just cut the middle layer to affect the mortice.
I make the pieces over size and then screw them together in the waste whilst it’s gluing up. This makes alignment and gluing far easier. You initially screw it tigether dry to get the fit right. If any joints aren’t ‘perfect’ you just saw down the joint, unscrew and realign and screw down again to get a perfect fit. I use Cascamite, long open times, and is completely water proof.
You need to make a template of the curve, (6 or 9mm ply / MDF) and after rough cutting the laminated top rail to approximate size, route or spindle mould to exact size using the template. I leave the ends long so I can screw the template to them. A track saw, or similar makes cutting the ends to length easy (I actually use a sliding table saw). Using the template adjusted, route or spindle mould a rebate to take the boarding.
If your using ‘cheap wood’ I would laminate the two styles again aligning growth rings opposing each other. You can again cut the motices if you like before gluing together. It’s just quicker with a hand saw and a chisel than using a mortise chisel afterwards.
I raise the bottom rail up about 2 inches off the bottom to help reduce rot / allow for fitting.
All the rail tops need to be chamfered at a minimum of 9 degrees. Make sure you allow for this when considering your mortice and tenon joints. Normally the tenons on the rails are actually half laps as the boarding takes up the ‘other third’ of a normal tenon.
Boarding should be tongue and groove. Paint or finish the tongues and groves before assembly. Paint / finish the rails and rebates before applying the boarding.
The central board should be ideally grooved on both sides. The reason for this is that should the gate need subsequent repair who ever is doing it will bless you as getting the boards off is far easier. Traditionally each board would be nailed on each rail and in the Rebate by a single nail positioned centrally in the plank. I actually usually use more in case any nails rot through and the planks come loose.
Here is a gate I knocked up that’s similar to yours. It had a double laminated head. You can see the template in the background. The workshops a mess as I’d just moved in when this was made / photo taken.
I think ive got the general idea of what you have posted. 
