Clock domain crossing of a gray counter. Any constraints / directives / cells?

Use TimeQuest. (You might be using TAN, which I recommend against.) Read the manual about TimeQuest first. Create an .sdc file with:

derive_pll_clocks -constrain_base_clocks

derive_clock_uncertainty;# (This line should be in there if targeting a 65nm or smaller device)

To cut the paths, either:

set_false_path -from *grayctr133 -to *grayctr50

set_false_path -from *grayctr -to *grayctr133

To do all paths between domains, do:

set_clock_groups -asynchronous -group {clk50} -group {clk133}

where the clock names come from what was created with derive_pll_clocks. This is most easily found when running the Report Clocks task in TimeQuest.