If Genesect EX uses "G Booster" to attack a Pokemon with Grass Weakness but that Pokemon has a Weakness Policy Tool attached, is Weakness applied or not?
G Booster isn't affected by "any effects on the Defending Pokemon", but the term "any effects" does not include effects that alter Weakness or Resistance. So in this case, Weakness is NOT applied.
Source: TPCi Rules Team (2015-02-26), TPCi Rules Changes Announcement (2017-05-03), TPCi Rules Changes Announcement (2017-05-18)

Relevant cards

Note: All applicable cards may not be listed below.
BW10_92
XY5_142
XY5_164