Parshas Ki Savo

Chofetz Chaim repeats this later in the Sefer:

It's clear from the Chofetz Chaim that speaking Lashon Hara on occasion does not put one into the category of a Baal Lashon Hara who loses his portion on Olam Haba, among other punishments.

However, it would seem from Onkelos not this way. The passuk says:

Many meforshim learn that this passuk is referring to the sin of Lashon Hara where one strikes his friend without him even "feeling" the blow at the time he is being verbally attacked, as Rashi says:

Onkelos renders the passuk:

Although the passuk uses a conditional present tense, ("One who strikes"), Onkelos changes the tense of the passuk to future tense, ("One who will strike").
The ArtScroll Onkelos suggests that perhaps Onkelos is making this change in order to clarify the meaning of the passuk. By using a conditional present tense, it can be taken to mean that the curse only applies to someone who regularly speaks Lashon Hara. To dismiss that notion, Onkelos puts the verse in a future tense to say that this is not the case, and even if someone speaks Lashon Hara just once, he is cursed.
(In order for this not to be a contradiction with the Chofetz Chaim, we need to say something along the lines of that the demographic who are included in the curse are not necessarily the demographic who are included in the many punishments like losing their Olam Haba.)

One could side step the question entirely by saying that Onkelos doesn't learn the passuk to be talking about Lashon Hara at all, and instead according to its most simple reading about striking someone literally and not according to the "homiletical" approach of Lashon Hara.

However, Biurei Onkelos has a diyuk in Onkelos here that he does subscribe to Rashi's approach. He is medayek from the omission of a lamed prefix at the beginning of the word חַבְרֵיהּ (not לְחַבְרֵיהּ) in contrast to where Onkelos does add the lamed prefix in the where the passuk talks about "smiting a man" in Shemos.

The Nefesh HaGer also learns Onkelos to be subscribing to Rashi's approach but from a different diuk. He gleans from the fact that Onkelos changed his translation of the word "makkeh" in our passuk from the identical appearance of the word "makkeh" in the immediately following passuk.
In our passuk he renders it as "strike" and in the next passuk he renders it as "kill."
Nefesh HaGer suggests that Onkelos renders makkeh in our passuk as "strike" in order to hint to another kind of "striking", even not of a physical kind, but the verbal kind, through Lashon Hara.