I'm creating my own card game and am currently designing the back of the cards I notice on a lot of packs there is a gap between the design and the edge of the card. Is this standardised anywhere?
It´s to avoid trimming essential text and images of the card when it's cut out after printing. Also, using black or solid colors can make the white card stock paper visible during the cutting process. Those infos you can find here.
However, some patterns, like this one, goes beyond the safe margin. So, the gap in the edge of the cards is just a best practice.
Card printers should provide a bleed, trim, and safe area. The trim area is essentially the size of the final card (I'll use 2.5" x 3.5", US Poker card). Typically you want to include an image bleed. The bleed means that your card background extends some amount beyond the trim line, for this example we'll say it's 1/8" (I don't know if that's a standard, but that's the only bleed I've seen). So if you want a card that is 2.5 x 3.5 inches with 1/8 inch bleed than the original card image to be printed will be 2.75 x 3.75 inches. The reason for the bleed is because the card cut will not be perfectly placed, so the printer is saying they can cut the card within 1/8" of centered.
For this same reason, the safe area will typically be the same distance in from the trim line, so the safe area of the example card would be 2.25 x 3.25 inches. Keeping text/images/card details within this area ensures that no necessary information will be lost when the card is cut.
Here is an example image of bleed, trim and safe area explained above.