Republicans complaining that the word treason is sometimes used in a sense different from the one in the constitution is rich... It sorta went mainstream with their last guy.

And, arguably, this instance is far closer to the tradition meaning of the word than, say, Barack Obama when he was labeled as such: treason is supporting a foreign enemy in war. Gabbard et al clearly have connections to and sympathy for Putin and people close to him. There's quite a bit of money changing hands, as well. The only formal element missing is that the US is not officially at war with Russia.