Hello and thank you for your question. I will be very pleased to assist you. I'm a practicing lawyer in England with over 10 years experience.
May I ask who would pay for the physical gate please? Would you share the cost?
There would seem to be two issues here to my mind. 1) the matter of ensuring that no rights of way are accidentally granted and 2) the potential for disagreement regarding the possible future removal of the gate and replacement with a standard fencing panel should either of you or any future owner decide they wish to change the gate back to fence.
It is very easy to ensure that you do not accidentally grant any rights of way. Providing you can evidence that the neighbours are accessing your land with your permission then no right of way can be formed unless expressly granted by you which obviously you would not do. this is because exercising rights with your permission takes place as a licence which can be withdrawn. Accordingly all you need do is consider writing a letter to your neighbour setting out the terms you have agreed and asking them to countersign it and you keep a copy. Your neighbours can do exactly the same vice versa. You may wish to mention how much notice must be given to withdraw the rights for the avoidance of doubt. Presumably you would not wish to have to give much if any notice.
The issue of changing back the gate to a fence should this ever be required is a little more difficult. If your neighbours are paying for the gate then it will belong to them (or any future owner should they sell). Accordingly if you wanted to change the gate back to fence then you would need to ask them to do so as you could not touch the gate without their permission. Therefore if this is of concern you may want to ask your neighbours to include an additional provision in their letter that they will replace the gate with a fence panel on request by you subject to xx months notice.
Is there anything above I can clarify for you?
Does the above answer all your questions or is there anything I can clarify or help you with any further?