How does one use Trackplug 5 as an expander? I’ve tried looking at the plug to see if it’s obvious, but at not point does the gate or either compressor offer less than 1:1.

The gate by definition is an expander, when the signal drops below a threshold the signal is attenuated further based on a certain ratio.  You could experiment with that, although you may find it easier to get desired results in MultiDynamics.

Unfortunately, the gate does not have the flexibility I need (you cannot set the floor higher than -inf) and Multidynamics does not have sidechain control.

I am trying to take a stereo room mic, have a little in the mix at all times, and then have it punch through slightly more (just a 1:1.3 ratio) whn triggered by the snare signal being sent from a sidechain.

A gate, though technically an expander, is not flexible enough for this. Saying a gate is an expander is a little like saying a limiter is a compressor.

Any suggestions?

You are definitely correct, a gate is an expander albeit a limited one. 

For sidechain capability we would either need to modify the gate component of TrackPlug or add sidechain to MultiDynamics, both of which are good ideas, so perhaps they will show up in a future update.

By the same token, you might add a “ducking” function at the same time.

Thanks-

Gary