Never had to treat the gore tex things that I have. Shouldn't ever have to.
On my old bibs I used to use Kiwi heavy duty water repelant, then I bought some Klim's, and don't have to worry bought it anymore.
Gore-tex, and all other waterproof + breathable fabrics should be treated with DWR (durable water repellent) when they are either saturating with water or not breathing correctly. All fabrics have this happen, there is no getting around this.
Snowmobilers, however, don't usually exert as much energy as backcountry skiers and thus don't feel the lack of breathability as often.... until we get stuck in deep snow! AKA, your average trail rider really only needs waterproofing, thus a waterproof fartbag (1-piece) is good enough for the majority of riders.
Saying DWR is bunk will get you called a jong in other corners of the internet
PS, I don't reccomend most ski equipment for sleds, the seats tend to eat up pants - you're better off with snowmobiling apparel.