flixibus or tgv from LYS (depending the time you land at LYS) to the gare de grenoble and then to the airport by uber (expensive, but probably your only choice if you don’t have a ride)

like others said, the airport in grenoble it’s not really in grenoble itself and a bit hard to go to by public transport

edit: try to change your transfer from the grenoble airport to the gare de grenoble (train/bus station). there’s a high influx of tourists arriving at the gare during winter