can i stick 6ga wire directly in earth instead of using grounding rod?

I'm sure (in the sense of "no way would that possible be allowed", not in terms of "actually having ever read the code") that's not allowed. If nothing else, a wire will not be secure in the ground the way that a rod will be.

Electrically it would be the same, but not physically, and since a ground wire/rod could be in place, not checked for physical issues, for many years, that is a real concern. A rod hammered into the ground is very secure, a wire is trivial to pull out.

"Buried wire ground" is possible, but that's a ground ring "not smaller than 2AWG, at least 20 feet long" buried 30 inches or more deep around the building.

If you are building the building, providing a concrete encased electrode (usually an electrical connection to the reinforcing steel) as part of the building footings is inexpensive and very effective.

You can stick your 6Ga wire a couple of feet in the ground and clamp it to a ground rod driven from the bottom of the hole (which makes it more effective) but no, you can't just stick a 6Ga wire in the ground and call it a day. If this is your only grounding conductor, you should extend a trench so that you can drive a second grounding rod a minimum of 6 feet (more is fine) from the first rod (it's far less expensive than doing the measurement with the fancy equipment you won't have to determine if one ground rod is adequate.)

You may also need to protect the grounding wire with conduit where it is "exposed to damage" aboveground.

If you don't mind mixing up some concrete there's one more type of electrode you could use, but you'll have to go one step heavier with the wire. A concrete-encased electrode in NEC 2017 250.52(3)(2) consists of at least 20 feet of bare copper not smaller than 4 AWG encased by at least 2 inches of concrete which is in direct contact with earth. The Code doesn't stipulate the shape of the wire or the concrete nor the depth of the concrete. It seems reasonable to think the concrete could be as thin as 4-inch thick flatwork on or below ground and the wire could be formed in a coil or zig-zag rather than be stretched out linearly.

A concrete-encased electrode is usually accomplished with 20 feet of 1/2 inch or larger rebar embedded in a footing or foundation and is often called a "ufer" ground. The 4 ga wire option above is given as an alternative to rebar.