Exactly they were not called straight away. Call the police or tamper with shutters &%+((£ Tampering with shutters won.
But how would Gerry know beforehand that the police were not going to be called straightaway? Someone outside of the group could have called them as soon as the alarm was raised. How did he know Mrs Fenn wouldn't immediately call them or Reception on hearing the commotion? How did he know in advance that there was not going to be a couple of waiters at their table when Kate raised the alarm - who might have immediately called the police - or neighbours or other passing holidaymakers?
How could he know there wasn't one - or even more police cars in the area - who could be at the complex within minutes after he left?
How could he know in advance that he wasn't going to bump into any number of holiday makers or locals during the time he was openly carrying a dead body through the streets? It wasn't as if it was in the dead of night when everyone was in bed.
How did he know in advance that when he returned and changed his clothing - none of the many people who were around by that time, would notice.
No-one in their right mind would devise a plan with so many unknown possible eventualities - all of which could be his downfall. Especially when there were far simpler plans he could have adopted.
IMO your theory fails PF - because it is based on hindsight of events which Gerry could not possibly have foreseen at the time.
If the open window and shutters were planned by Kate and Gerry to be the proof of an intruder - why did he close them before the police arrived? That makes no sense.