In the following code that returns a list:
public List GeAllCust()
{
var results = db.Customers
.Select(x => new { x.CustName,
You are selecting to an anonymous type, which is not a Customer.
If you want to do (sort of) this, you can write it like this:
return db.Customers.Select(x => new Customer { Name = x.CustName, Email = x.CustEmail, Address = x.CustAddress, ContactNo = x.ContactNo }).ToList();
This assumes the properties on your Customer object are what I called them.
** EDIT ** Per your comment,
If you want to return a subset of the table, you can do one of two things:
Customer as I specified above, or:List (assuming ShunkenCustomer is the name that you choose for your new class.)