Not a lot of info to go on, but a lift gate is very easy to build. The drop down gate is a little tougher to build because you have the latching mechanism and you must source the parts for those. Springs can be found at any hardware store. You will have to test it to be sure you get proper compression to be able to push the locking mechanism OUT with enough force, but not too much. Likewise, piano hinges are recommended.
To set the pressure for the locking mechanism, you can just affix a fine tuning blocking board to the top of the flip lock push panel to set the length in the up/lock position. You would place it approximately where you see the upper dark shadow in the image. You would place the door in the closed position, then attach the fine tuning block. You can bevel the top slightly for a smooth locking fit. A little furniture wax helps too for where the two wood surfaces meet.
I would recommend a 1×4 pine board. This gives you the perfect pressure without having to be exact on the push panel position.
Check out the Flapper Door page for piano hinge sources.