Gateway 3ds blue ds card is compatible with 3ds 6.1 system and backwards compatible with any lower version. It allows you to play ds roms on 3ds console.
Gateway 3ds red 3ds card is compatible with 3ds system version higher than 4.1 but lower than 4.5, which allows you to play 3ds roms on 3ds console.
Please follow the setup guide step by step for your gateway 3ds flashcart.

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Best Place to buy Gateway 3ds flashcart: 
US Seller: http://www.gamezway.com/gateway-3ds-play-backup-3ds-roms-with-gateway-3ds_p18856.html
UK Seller: http://www.digimartz.com/gateway-3ds-supports-all-3ds-roms_p1136.html
DE Seller: http://www.bestekauf.com/3ds-flashkarte/394-gateway-3ds-roms-flashkarte-fur-nintendo-3dsxl.html
FR Seller: http://emonstre.com/gateway-3ds-pour-nintendo-3ds-roms.html
JP Seller: http://www.aceshop.jp/product-p-771.html
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