lupus, вот здесь нашёл какую-то тулзу:
GXTConvert v0.6
Скачать можно в секции
Releases
GXTConvert is a somewhat rudimentary converter for GXT-format textures used by games for the PlayStation Vita. It is written in C# and based on the .NET Framework.
GXTConvert is currently only receiving limited maintenance. Please see the Scarlet project for a more generic set of libraries and a sample converter application to convert various game or console image formats to PNG, including GXT.
Gravity Rush (*.gxt)