How long can one live after being diagnosed with a gist if he doesn't remove it?

Variable. Most all gist tumors are benign. But they will likely grow over time, and there is a risk that they can become malignant or cancerous and spread to other organs such as the liver. I do not know how long one can live with this tumor, but it is best not to wait for it to get too large. There is also some medical oncology treatment for these (gleevec).