The package is comprised of services used to manipulate various adwords structures. To access a service you need to create an gads.Auth and parse it to the service initializer, then can call the service methods on the service object.
authConf, err := NewCredentialsFromFile("~/creds.json")
campaignService := gads.NewCampaignService(&authConf.Auth)
campaigns, totalCount, err := campaignService.Get(
gads.Selector{
Fields: []string{
"Id",
"Name",
"Status",
},
},
)