Since when Gowther become one of Ten Commandments? I know that Gowther is one of Seven Deadly Sins, but in later manga, it is revealed that he also one of Ten Commandments.

So, since when he is become one of them? Did he join Ten Commandments after or before he join in Seven Deadly Sins?
It has already been 3 thousand years with respect to the current events when the 10 Commandments fought for their race and were defeated and sealed away. Meliodas himself being a demon was definitely involved and witnessed it all. He has been alive all these thousands of years.

The Seven Deadly Sins were formed much more recently. The Seven Deadly Sins were an independent group of knights serving the Kingdom of Liones. So it is obvious that they have been around for a while but only during the time of Liones.

Gowther was recently revealed to be a doll created by a great magician. Meliodas is a demon. King is a fairy. Diane is a Giant and Ban is an immortal. Those explain why they have such long life spans. But the true nature of Escanor and Merlin has not been revealed yet.

Still, the Kingdom of Liones was probably founded much later after the 10 Commandments were defeated. The back stories of some of the characters have been revealed before they joined The Seven Deadly Sins. Ban and Diane for instance. They have been shown to be in their younger years living in pretty much the same timeline as the current one with no demons. So it is safe to assume that The Seven Deadly Sins were created fairly recently as compared the thousands of years.

Gowther is revealed to be "Gowther the Selfless" of the 10 Commandments. He lost his memories because of his selflessness. By the time he joined the Sins, he had no memories of his past. That means he must have been a part of the 10 Commandments before he had joined the Sins.

It is unclear though whether he was part of the 10 Commandments back when they were defeated 3 thousand years ago or became a part of them afterwards like Fraudrin. If Gowther was a part of the 10 commandments back then, then Meliodas probably knows about him and hasn't revealed the truth to anyone thus far.

UPDATE:
Some of Gowther's past was revealed to King and Diane by Gloxinia, the former fairy king and Dolor, the former giant king. Gowther himself was a part of the 10 commandments long before the war between the clans even started. However, he had been imprisoned by the Demon King. The Gowther we know is just a doll created by the real Gowther, released into the real world to be his eyes and ears.