Well, that is what some troops called it!
The correct title was "Cap, General Service". It was first introduced under "Army Council Instruction 1407" of September 1943. This cap was designed to replace the "Field Service cap" or "FS Cap" that had been worn since the outbreak of war. (Familar to all as the cap worn in TV's "Dad's Army") I have coverd the FS cap here:
These caps were issued in priority to units serving overseas. UK based units got theirs later on. It was at first unpopular due to it's over large appearance. The cap was not a beret. It was made from several peices of drab cloth material, whereas a beret was a one piece item. It was based on the Scottish balmoral bonnet in design. First issues were made from the same gaberdine cloth as the old "FS" cap.
Badges worn on it were the conventional Officers bronze, and OR's badges in both plastic and metal. Units which had special distinctions could still wear these on the "GS" cap.
The cap was worn by regulation one inch above the eyebrows, with badge over the left eye and the cap pulled down to the right. But many wartime photos will show it worn pushed back on the head, which seemed to be a fashion with many squaddies late in the War.
Officers could only obtain the cap upon repayment to the RAOC. They were not allowed to purchase the cap until their unit had been issued with it wholesale. Higher rankng Officers often got away with wearing a Khaki beret, which was, of course, against regulations Fashion conscious OR's would also risk wrath from upon high by buying one of these for "walking out".
Interior pic shows the Glasgow makers mark and 1945 date