This gives you all the selected assets:
TArray<FAssetData> AssetDatas;
FContentBrowserModule& ContentBrowserModule = FModuleManager::LoadModuleChecked<FContentBrowserModule>("ContentBrowser");
IContentBrowserSingleton& ContentBrowserSingleton = ContentBrowserModule.Get();
ContentBrowserSingleton.GetSelectedAssets(AssetDatas);
The problem with GEditor->GetSelectedObjects() is that it doesn’t include assets that haven’t been loaded yet, even if they are selected in the GUI.