What GDI unit is the assault soldier's counter?

Do you mean the Militant? If so, then low-cost counters are:

Rifleman - same level and has the defender advantage or 2v1

War Dogs

Rhino

Drone Swarm

Talon

Mid/High cost pretty much everything strong against infantry.

I recommend always open with rifleman/war dog to scout.

no, I mean the lab

Flame troopers or shockwave troopers the new gdi scatterer uhh flame tank uhh the gdi disruptor and artillery