Agnes was Princess of Navarre and a close relative of the english royalty.
She was second cousin to the Black Prince and great granddaughter to Henry III. As such she was undoubtably part of Gaston’s plan together closer to the English crown.
Gaston’s marriage to Agnes was however a clever choice and typical of Gaston’s political aptitude.
She was also the grandaughter of King Lois X of France. this contact could have been used if the negotiations with the English had failed; which indeed they did.
The marriage was an unhappy one. They had one son. who Gaston stabbed to death after an acrimonious separation.