Thanks to @Diego who opened an issue with geopandas (https://github.com/geopandas/geopandas/issues/3433), I came across two different ways to solve this problem (which originates from pyproj):

Option 1: Disable network use

As described at https://github.com/pyproj4/pyproj/issues/705, add these lines to the preamble:

import pyproj
pyproj.network.set_network_enabled(False)

Option 2: Pre-download projection data

As described at https://pyproj4.github.io/pyproj/stable/transformation_grids.html, add the proj-data package to the conda environment. It's also safest to use a single channel. Either run conda install -c conda-forge proj-data or use this environment:

name: geotest
channels:
  - conda-forge
dependencies:
  - python
  - geopandas
  - proj-data