The Gun Dragons are a series of DARK-Attribute, Machine-Type monsters. This series focuses heavily upon the usage of Coin Tossing to activate their effects of destroying your opponent's cards on the field. The original Gun Dragon monsters are ones that need to Tribute Summon (although "Gatling Dragon" can only be Fusion Summoned whilst "Twin-Barrel Dragon" is the only monster in the series that can be Normal Summoned without a Tribute.) The appearance of this series seem to be based off firearms (although the TCG censored the fire weapons changing it to lasers) and share 'dragon' in their name though they aren't of the Type (this is can also be seen with the Cyber Dragon cards).
As noted above, a Gun Dragon Deck will focus upon getting the needed advantage during a Coin Toss so using "Second Coin Toss" will be useful to increase the chance of activating their effects. Being Machine-Type monsters, Gun Dragon monsters should also be supported by Machine support cards such as "Limiter Removal" and "Machina Fortress".