I Discovered That My Great-Grandfather Was Adopted In The Early 1900s. Do I Stop Researching?

That depends on you and your relationship with your existing relatives.

If you wish to pursue that particular line because of interest, curiosity, or a gift to the remaining biological relatives, by all means, continue your research. You will understand, though, that you have no biological connection to that particular line passed the adopted child.

However, whomever they married (unless they were also adopted) will still be your biological ancestors, and you may wish to pursue that line in more detail.