Selecting different values from different columns
It's not very clear what you are after but here's a query under my understanding that you want to find the last timestamp where queues = 'ACS' and the last timestamp where queuesname = 'GHK' for every groupnumb:

with grp as
  ( select groupnumb, max(timestamp) as timestamp
    from tableX
    where queues = 'ACS'
    group by groupnumb
  union all
    select groupnumb, max(timestamp)        
    from tableX
    where  queuesname = 'GHK'
    group by groupnumb
  )
select min(timestamp) as min_timestamp,
       max(timestamp) as max_timestamp,
       groupnumb
from grp
group by groupnumb ;