The eight keys in the middle panel A2 are all the same size. Therefore you can use a GridLayout with two rows and four columns in it. However this isn't true for A1 and A3, so let's continue. Split panel A1 into a grid of two rows and one column. The top cell will be used for the TextField display. However the bottom cell still needs to be split into four pieces so add Panel B1. The bottom is more complex still. Split it into a grid of one row and two columns and put panels B2 and B3 there.
Panel B1 contains four keys, all the same size so put a grid of one row and four columns there, and add those keys. Next split Panel B2 into two rows and one column. Put the oversized zero key in the second cell and Panel C1 in the first. Next split B3 into two columns and one row. Put a panel in the first cell and the oversized equals key in the second.
Finally split C1 into two columns and one row and put the one and two keys there. Then split C2 into two rows and one column for the three and decimal point keys.