While I cannot tell for everyone, I still share my experience. Whether or not you will require a strong GPU strictly depends on which classes are you going to attend. Algorithmic classes, as well as those that focus on programming languages as such does not require any specs and could be easily done even on the potato. Of course, you will have to wait a little bit longer for compilation to finish, but it is not impossible. GPU then is irrelevant, I never had a need to use external GPU.
However, I once had a situation when I could not finish my Mathematica project due to lack of power, and I believe if you deal with graphics tools then you will need a good graphics card. But still, university should give you an opportunity to use their PCs as it is not a requirement to own a powerful laptop.
My opinion is that you do not need GPU at all to major in computer science.