You play Halo Wars: Definitive Edition. Pick the new UNSC leader, the Gundam Commander, and field mobile suits and ace pilots taken from your own copy of SD GUNDAM G GENERATION CROSS RAYS.
What you get
Mobile suits instead of tanks at the Vehicle Depot: Graze, GN-X and Leo Ground Type. Their names and their HP, attack, defense, mobility and main-weapon power are read from your Cross Rays files. Halo Wars hitpoints, damage, speed and cost are scaled from those numbers.
Three ace pilots as hero units: Heero Yuy in Wing Gundam Zero (your starting unit), Kira Yamato in Strike Freedom Gundam and Setsuna F. Seiei in Gundam Exia, with their Cross Rays pilot stats. Every tooltip shows the Cross Rays stats it came from.
AI commanders can pick the Gundam Commander too, so mobile suits can fight mobile suits.
The Cross Rays soundtrack plays from your copy while Halo Wars runs. Turn down Halo Wars' own music in Options to hear only Cross Rays.
Single player: skirmish against the AI.
The units use the Halo Wars models of the units they replace (Scorpion, Cobra, Wolverine, Spartan), because Cross Rays' 3D models can't be converted to Halo Wars reliably. Nothing from either game is included. Each time you play, it reads what it needs from your installs. The first Play takes about an extra minute while it unpacks Halo Wars' data.
Credits
KSoft.Phoenix / PhxTools by KornnerStudios (MIT): reads Halo Wars' archives
SharpZipLib (MIT)
VGAudio by Alex Barney (MIT): decodes Cross Rays' music