The book you have read is talking nonsense.

There has never really been the 'siesta culture' in the business community in Gibraltar. Not in offices, not in retail either. Not in the past and not in the world of today either.

As in most of the world, there was a culture of businesses closing for lunch in years gone by, but not long 'siesta periods' and then open late in the evening. Just an hour or so. As with most of the developed world, businesses are now open through lunch.

What there is - but only for some government departments - is 'summer hours' situation. In the summer months of the year, employees have the option to arrive earlier, work straight through (or with a shorter lunch break mid morning, etc) and then leave in very early afternoon.

But this is purely some government departments - and the business comunity (and it's workers) don't like it! - so the government needs to ensure all public ensure, etc remain open, as businesses are working 9-5/6 year round.

(schools also adjust their hours a bit in summer, to start and finish earlier).

But there is certainly no siesta culture. (Aross the border in Spain, there was a culture - and still is to an extent - to close early afternoon, and reopen into the evenings. And smaller shops in Spain do this today - being open quite late in the evening, but close in the afternoon.
This is NOT the case in gibraltar, most shops in Gibraltar will be closed by 7pm, business by 5.30pm. The odd shop later.

The evening meal culture in Gibraltar is also a bit earlier than across the border in Spain.