GridBaseline is an option for GridBox which specifies where the baseline of the grid represented by the GridBox should be assumed to be.
GridBaseline determines how a GridBox will be positioned vertically with respect to other boxes, say in a RowBox.
GridBaseline->pos specifies that position pos in the GridBox should be assumed to be the baseline of the GridBox and should therefore be aligned with baselines of other boxes.
Possible settings are:
See The Mathematica Book: Section 2.8.11.
See also: RowAlignments, AdjustmentBox, CellBaseline.