So you're replacing an oil pressure sensor in the cylinder head?
When you say you've rounded off the thread in the block, do you mean the thread has stripped out as you've removed the sensor?
If so; ideally a helicoil would need to be fitted. This involves cutting a larger thread (casting permitting) and winding a threaded insert with the correct internal pitch into the now oversized thread. To do this job you would really want to do it on the bench and be sure all swarf is cleaned out of the head.
Either remove head and have a go yourself if you're a confident engineer or take to a machine shop to have the job done. Other option is the change the cylinder head itself. The polo GT shares the same cylinder head casting (although the exhaust valves differ on a G40 and the camshaft is G40 specific, the internals can be swapped however). The part number is on the exhaust side of the head iirc and ends in a G.