So...huge?

Damn. I thought I could get the job done, but the requirements are the double to what I have.

That's wrong, it calculates the surface distance not the distance through the earth, while claiming otherwise. From the geopy.distance.great_circle documentation:

Use spherical geometry to calculate the surface distance between points.

This would be a correct calculation, using the formula for the chord length from here:

from math import *

# Coordinates for Atlanta, West Georgia
atlanta_coords = (33.7490, -84.3880)
# Coordinates for Tbilisi, Georgia
tbilisi_coords = (41.7151, 44.8271)

# Convert from degrees to radians
phi = (radians(atlanta_coords[0]), radians(tbilisi_coords[0]))
lambd = (radians(atlanta_coords[1]), radians(tbilisi_coords[1]))

# Spherical law of cosines
central_angle = acos(sin(phi[0]) * sin(phi[1]) + cos(phi[0]) * cos(phi[1]) * cos(lambd[1] - lambd[0]))
chord_length = 2 * sin(central_angle/2)

earth_radius = 6335.439 #km
print(f"Tunnel length: {chord_length * earth_radius:.3f}km")

A straight tunnel from Atlanta to Tbilisi would be 9060.898km long.

Depends what the great circle function does. But chances are it gives you the arc length between the two points, which would give you the shortest distsnce between them on the surface of the earth.

If you wanted the distance through the earth you would need to work put the central angle of that arc, then use that to work out the remaining edge of the triangle formed between the two cities and the centre of the earth.