It takes serious engineering. And it's been done.The Dragon, from Zollner Elektronik AG.[1] About a third of the size of Gundam, mostly so it could be transported by road.
True quadruped. Best engineering in the giant robot space so far.
The Sultan's Elephant [2] was bigger, but less technically advanced. The headaches of transporting it were too much of a problem. Once your art project gets bigger than what can be moved by road or rail, there's a problem.
The Land Walker, from about 2006, was about a third of the size of the big Gundam. Biggest walking biped to date.[3]
There's also Eagle Prime, which is really a tracked vehicle.[4]
[1] - the dragon is rolled around on a wheeled platform. There is one clip of a leg lifting, but none of it actually walking.
[2] - The Sultan's Elephant is moved around on wheels. It doesn't walk.
[3] - That thing is "skating" around on wheels, again not actually walking.
[4] - And yet another machine that doesn't walk but actually rolls around on treads.