"I thought" here means that the speaker was mistaken. "You had given up your job" is in the past tense because it's not true here (hypothetical). Other examples of this structure include "I thought you were sick today" (but I can see now that you look healthy), "I thought you didn't have internet access" (but you are returning my messages), etc.

Both sentences are grammatically correct. They are only different in the tense.

The most natural way to say it is : I thought you had given up your job. But you can say I think you have given up your job.