Difference between await and GetAwaiter() in C#

I have following methods which are working fine for me:

1.This is using "async await" approach:

public async Task AddClients(string clientId)
{
   bool isAlreadyAdded = await CheckIsClientExists(clientId);
   if(isAlreadyAdded)
   {
     await AddNewClientToGroup(clientId);
   }
}

2.This is method is using GetAwaiter() approach:

public async Task AddClients(string clientId)
{
   bool isAlreadyAdded = await CheckIsClientExists(clientId);
   if(isAlreadyAdded)
   {
      AddNewClientToGroup(clientId).GetAwaiter().GetResult();
   }
}

I have seen the link : Async await vs GetAwaiter().GetResult() and callback . And this have not explained why and how GetAwaiter() leads to a deadlock situation.

Can anyone explain the difference between the both approaches, i.e await and GetAwaiter()?