The Chofetz Chaim says that loshon hara is equal to all sins combined. He does quote the Yerushalmi 1:1 which says, “ותלמוד תורה כנגד כולן”. The Yerushalmi does not say ולשון הרע כנגד כולם. Although, he does say that loshon hora is equal to the 3 cardinal sins.
However, where does it says that loshon is equal to ALL SINS combined?
The Yerushalmi Peah 5a does say ולשון הרע כנגד כולם, however I hear your question, that from the context of the Yerushalmi is would seem that Lashon Hora is equal to the three cardinal sins, but not to ALL sins? However the Chofetz Chaim is really only saying this over from the Vilna Gaon in his Igeres ומה לי להאריך בזה העון החמור מכל העבירות כל עמל אדם לפיהו אמרו חכמינו ז”ל שכל מצותיו ותורותיו של אדם אינו מספיק למה שמוציא מפיו. That all of a person’s mitzvos and torah learning will not equal to what a person says from his mouth”. Also see Shnos Eliyahu Peah 1-1 on the words V’havoas Shalom, that there are four positive mitzvos mentioned in the mishna, kibud av v’em, gmilus chasadim, havoas shalom, and talmud torah. On the negative side there is idolatry, murder, arayos, and lashon hora. The Gaon says that these four mitzvos are juxtaposed to these four aveiros, with Lashon Hora being the opposite of Talmud torah, and since Talmud torah is כנגד כולם, in schar, so too לשון הרע כנגד כולם on the negative side.
Yerushalmi Yerushalmi Peah 5a