The terms are confusing. In most cases for pension/retirement plans your taxable income is the same as your gross. But not always. It might not all be taxable depending up the type of plan and especially upon whether you got a tax-deduction at contribution time. If you made any contributions to the retirement plan with "After tax" money, you get those back tax-free (so in box 1 but not box 2a) bit by bit over the years. But it is usually up to you to track that, which is why Box 2a is usually empty and 2b is usually not checked.
If box 2a has a number and 2b is checked and it seems right then use it.
(Super-technical: box 1 is gross distribution not gross income. Elsewhere on your 1040 gross income is totaled up and many things are keyed off of that. But for retirement plans return of after-tax contributions is not income at all. It's a return of your own previously-taxed money, not a gain of any sort. So box 1 is not labeled income but rather a distribution.)
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