I had the same issue. Unfortunately GL is not helping at all. Instead, they claimed that tourist should not be enjoying such “special offer” for tax refund. I had shop so much in EU and it looks like only GL is giving such dispute and it seems norm to most tourists.
I hope to resolve this soon too with GL. I have contacted EU tax and GL should be able to return the charges from my credit card however, according to GL they must have the original copy which is impossible for me to produce since it has been sent out long ago.
I hope you have took an image of your tax form and seek for other relevant help regarding this.