Married, filing jointly. Retired.

Do we have to file taxes if we've had no income for the past several years (which we did not file for those past years), but this last year, we still had no income, but did have capital gains from sale of mutual funds of ~$13k?

I assume that your filing status is MFJ. Then, no. your taxable income is zero as MFJ, your standard deduction and personal exemption of $20,300 ,already exceeds $13K of capital gains from sale of mutual funds.