I mean, I literally can't see why someone would be mad that she's asian besides racism... When it's black actors, well, I disagree with it, but I can understand the reasoning as they do look different from the originals in that way. But with Gemma, not really. Sersi and Gemma are both pretty, young, skinny/fit, dark-haired and, even the skin color itself, it's not like Gemma was a dark-skinned asian. She looks the part just fine.
I honestly haven't seen backslash against her casting anywhere else. With the characters that were genderbent, yeah, and even with some who weren't too. But with Gemma not really.