Viper School Gear is set from base game The Witcher 3: Wild Hunt and from its first expansion Hearts of Stone based on school for witchers called School of Viper (from which came Letho of Gulet for example).
In the basic game the set is in fact only two low leveled swords (one silver, one steel) previously owned by Kolgrim from the Viper School. When you collect the first weapon schematic, it will start a new Scavenger Hunt secondary quest.
If you buy the expansion Hearts of Stone expansion, you can a full set of Viper Gear. The armor looks like the armor that Geralt starts the game with, but the stats are different and the armor has a snakeskin pattern on it.
There is no secondary quest associated with this gear, meaning that it can be completely missed. The diagrams for the armor is bought from Countess Mignole during the Open Sesame! (Hearts of Stone) quest, while the steel sword diagram is found within the Borsodi Auction House vault later in the quest. The silver sword of this set has no crafting diagram, but the forged blade can be found within the puzzle world of Gaunter O'Dimm.