Use wood posts for the gate frame. I used metal post anchors sunk in concrete so the posts don't rot. My clothesline has been up 30 years anchored in this manner, works really well.

Think door jamb like in the house. Hang gate even with hinge side of gate and seal and stop the gate with wood strips attached to the inside of the gate posts. If you lay large concrete blocks at the threshold then you can add wood to the bottom of the gate to fit or you might prefer to do overlapping wood like on the sides but that would leave a bit of a step over.

Welded wire naturally wants to return to the coil that it was in. I just overlap the gate edge (latch side) and secure with a fiberglass rod. The hinge side has no hinge--only a continuation of the welded wire. This is pretty difficult to explain and a picture doesn't help. Let's imagine that the point where the fence starts is the latch side, where there is a t-post. Attach the welded wire to it. Now put in a t-post where the hinge side will be. Let's assume you want a 3' gate. Overlap the strike post by 6-12 inches and then work backwards installing fence and t-posts.

Next year I'm going to attach a length of 1/2" PVC pipe with cable ties to the latch edge of my fence and secure It with the fiberglass rod through the pipe and into the ground. That should keep the edge taut and not leave an easy entrance for the varmints.

Look into fence construction for the gate. 2 2x4 or 2 4x4 cemented or dug into the ground for the frame, then follow a shed door pattern. I use a flapper at the bottom of my gate. You could make it level and brick under or put weed barrier guards under the gate in combination with a level ground and flappers.

Another thing on mine is the gate opens outwards and the ground inside is higher then the ground outside the gate which doesn't leave gaps. You'll probably want a door latch at top and bottoms of the gate to prevent Prying and or shifting. Tends to shift over time due to tilling and uneven watering.

I was having a hard time deciding on how to make a gate on my garden several years ago. For the fence, we put up mostly heavy duty t posts, but sank 4x4 wooden posts at the corners and on each side of 2 gates. You could do this with just metal posts too.......but what I ended up doing was taking a section of a cattle panel, just wide enough to cover the entrances to the garden and attached utility wire to it, but also put some chicken wire at the bottom, since rabbits can get through utility wire. You have to bungee it to secure it to both side posts. You can lift the gate to completely remove it, or slide it to one side, or you can just open one side wide enough to walk through. It has worked well for me.