Arsenal Summoner was made to be a search card for the "Guardian" archetype which was released with Guardian cards Baou, Ceal, Elma, Grarl, Kay'est, and Tryce, and then later Dreadscythe and Eatos
Base Set was one of the most popular sets ever but these misprinted cards were limited, and nowadays, many collectors will do anything for these versions
It throws another +1/+1 counter on each of your modular creatures each upkeep
Gyarados cards are, on the whole, pretty terrifying this is aided by the fact that Gyarados is, on the whole, pretty terrifying
It probably makes sense now, why a Gem Mint PSA 10 Gretzky OPC rookie is worth almost 20x that of a perfect 10 Jordan rookieits that there are only 2 Gretzkys out there, making it a much harder card to find in Gem Mint condition