Delete the instruction "If you don't have PyTorch installed, follow their instructions...". Exactly as the installation instruction suggests, I ran
pip install torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cu124
and then
pip install "pix2tex[gui]"
which redownloads the exact same version of torch. (Why?? I have no idea. Because of the custom channel?)

Then I deleted the whole venv and started over. I ran
pip install "pix2tex[gui]"
which downloads torch as well (the correct version, with CUDA support). Finally, I ran the GUI, tested it on a math expression, and made sure that it used my NVIDIA GPU (instead of the CPU).