I’m not sure why you can’t get your daughter the discount. GM has a web site devoted to employee discounts:

https://www.gmfamilyfirst.com/ip-gmemployee/

and according to the site, “* Eligible employees and retired employees are able to sponsor their spouses, children, grandchildren, parents, stepparents, grandparents (including in-law and step), siblings (including full, half and step), stepchildren, stepgrandchildren, mother-/father-in-law, sons-/daughters-in-law and brothers-/sisters-in-law. Same-sex domestic partners (SSDP) of eligible participants also qualify. Eligible SSDPs are treated the same as spouses and, therefore, their eligible family members are also included.”

There is also a “contact us” area on the site that could better answer this question.