G1T

G1T is the texture format made by Koei Tecmo Games. It is heavily based on DDS texture format. Here is a table of most DDS formats:

Gust Tools
Gust Tools is a set of commandline utilities designed to work with Koei Tecmo game assets, including G1T. To unpack a texture simply drag and drop G1T file on gust_g1t.exe, and to repack do the same with the folder that it creates when unpacking.

Project-G1M
Project-G1M is a Noesis plugin for many Koei Tecmo's files. The usage can be seen in the GitHub readme.