Sidechaining in REAPER one of the most straight-forward things I’ve ever seen in any DAW!
Just click and drag the route button into the target plugin like this:
No need to do the complicated way of opening the routing window and start mapping things to channels. This pretty much sends the track into another track as the 3/4 channel ✨
If you want to sidechain the kick with your bass gate, just click and drag the kick’s route button into your bass gate.
Here’s how that would look like 👇