First, LB has officially unfollowed the embattled rapper -- as of Saturday, it appears Gunna no longer shows up as one of the 970 people Lil Baby is paying attention to on Instagram ... which might seem petty, but carries weight considering he *was* following until recently.

An unfollow in 2023 is the equivalent of telling someone ... I don't wanna be your friend anymore. That's where things seem to be between LB and Gunna, which is a big deal -- these two have been incredibly close for years. They put out a mixtape together once, and have rapped on each other's songs back and forth ... plus, they're both from Atlanta.

Now, on to the more transparent falling out ... Lil Durk clearly called out Gunna in a new track he previewed -- which features a line that alludes to a joint song they were on.

The video shows Durk and co. vibing out to the unnamed song, which features a quick line toward the beginning that goes ... "What happened to Virgil // he probably gonna tell." That's a reference to a song Durk did with Gunna of the same name ... so yeah, it's a shot.

As we reported ... Gunna's already under fire from a lot of people in hip-hop, including YSL members and associates -- who made it clear online that they're not messing with him.

This, of course, comes after Gunna was released from custody when he copped a plea deal with state prosecutors in the sweeping RICO case that saw not only him arrested ... but Young Thug as well. YT's on trial right now, and Gunna seems to be perceived as a snitch.