His condition deteriorated and he died before an ambulance arrived.
A post mortem examination found a collapsed lung, bleeding in the chest cavity and eight fractured ribs.
Dr Gartland told Coroner Ian Smith he did not order an x-ray because a high percentage of rib fractures were not seen on simple x-rays and there was no specific treatment for isolating rib fractures, the findings said.
He told Mr Wilmshurst to return if symptoms deteriorated.
But an independent doctor, Dr Charles Freeman, who provided an expert review of Mr Wilmshurst's treatment told the inquest Dr Gartland "seriously underestimated" the injuries suffered, the findings said.
If Dr Gartland had ordered an x-ray, the rib fractures would have been "clearly visible" and would have alerted the doctor to potential complications, Dr Freeman said.
He also noted that the centre was an urgent care facility and not an emergency department.
Coroner Smith recommended that clearer signs at the medical centre needed to be erected, advising patients the centre was not a hospital emergency department.
That also needed to be explained to patients on entry, he said.
The facility should also consider applying the latest version of the "Advanced Trauma Life Support Course Manual for Doctors" noting the "pitfall" of underestimating the severe repercussions of rib fractures, particularly with older patients, Coroner Smith said.