If my opponent used Gardevoir EX's "Shining Wind" attack to gain "no Weakness" and I play Pokemon Ranger when it is my turn, does their Gardevoir EX have a Weakness again?
Yes. The ruling about effects on Weakness and Resistance not being ignored has to do with damage calculations [see "General Gameplay -> Weakness and Resistance" section for reference], not about whether a Pokemon HAS Weakness or Resistance. Pokemon Ranger removes attack effects, including this one; so yes, Gardevoir EX would have Weakness again after using Pokemon Ranger.
Source: TPCi Rules Team (2018-11-15)

Relevant cards

Note: All applicable cards may not be listed below.
XY11_104
XY11_113
XY5_105
XY5_155