The whitepaper defines the exact syntax:
al/align alignx [aligny]
Going on with:
The alignment can be specified as a UnitValue or AlignKeyword.
So, for centering the whole block along both axis, using the AlignKeyword you need two parameters:
new MigLayout("al center center, wrap, gapy 20"); // centers in both directions
Next sentence:
If an AlignKeyword is used the "align" keyword can be omitted.
Which would be:
new MigLayout("center center, wrap, gapy 20"); // centers horizontally only
doesn't work though, looks like a slight glitch in parsing the parameters.