Use waterproof glue, make the joint tight-fitting, and leave the screws on the shelf. A lap joint is one of the strongest of all the glued wood joints, and a well-fitted glued lap joint will be at least as strong without the screws as with them.
But... there's a condition. How wide are the two planks forming that joint? And which wood is being used? I ask because of wood's inherent expansion/contraction with humidity/moisture changes. If the joint is too wide for that species of wood, and the wood isn't extremely well sealed against moisture intrusion, then expansion/contraction will destroy the joint; nearly all woods expand & contract across the grain far more than they do along the grain. As the wood becomes moist, it'll become wider but not longer.
Certain woods have a very low expansion/contraction rate. Other woods have a very high expansion/contraction rate.
If the wood is extremely well sealed, its inherent expansion/contraction will be minimized, but can never be totally eliminated (there's no such thing as a perfect seal).
If you're using wide boards, and that species of wood is subject to high expansion/contraction, then neither glue nor screws will prevent the joint from failing. Better in such a case to make a different type of joint, like a dovetailed saddle joint pinned across the "inside" corner, for example. That's much more difficult to make, though. Another option would be a long mortise with a tenon narrower than the length of the mortise.
Good outdoor gate joints are difficult things to make, since they're exposed to the weather and tend to collect pockets of water after a rain.
If the hinges are on the same vertical line, then (as noted by @DA01) the hinge should stay wherever you leave it (absent wind, etc). If the hinges aren't vertically aligned, then the gate will want to swing towards a specific point. You can use this to solve your problem.
Imagine that the gate is swinging on a rod, and can go 360° around the rod. If the rod is vertical then you can put the gate anywhere you want and it will stay. However, if the rod is leaning towards a certain direction, then the gate will tend to swing until it is pointing in that direction (because the gate will be lower at that point).
What if you move the gate to the exact other side of the rod from where it wants to go? It will sit there, balanced. If you then move it in either direction it will continue to swing in that direction until it reaches its low point.
So, what you want to do with your gate is to align the hinges so that the hinge line leans away from the the middle of the gate's range (at 45°). That way the gate will tend to swing away from the 45° point, towards the nearer of 0° and 90°.
Edit: When you do this, you may have to readjust the position of the gate on its hinges in order to have the gate properly fit into its opening. But, once you adjust the post side of the hinges to get the right swing, changes to the gate side of the hinges won't affect the swing, so you won't have to readjust anything.
Best Answer
In an attempt to answer some of the questions raised in your posting let me respond in answer format.
1) It can be advantageous in some situations to affix a timber post to the brick or concrete posts/walls in either side of the gate opening. This post can then serve as a better medium into which to attach hinges and latches.
2) A 3.8 meter wide gate will develop a huge amount of lever arm out at the latch end of the gate and this needs to be taken into account when developing the gate design.
3) The size of wood used can play a big part in how heavy the gate will end up being. The type of wood can play a role as well. This is one reason that boards are often used for gate facing instead of thicker planks. Cedar wood is also much lighter than hard wood like oak and thus becomes a popular choice for gates.
4) Adding appropriate bracing into the design of the gate will keep the gate from sagging.
5) Successful wide gates can often benefit from making the aforementioned side posts much higher than the gate opening and then using a cable from the top down to the end of the gate away from the hinges to help support the weight of the gate. Sometimes you can see the tops of the two higher posts connected across by a horizontal member. This provides support from post to post to keep the weight of the gates from pulling the posts to the sides.