What affects the value of Gyruda, Doom of Depths
In non-tournament play, its simply a card you own thats not in your starting deck
for that battle, your monster cannot be destroyed by battle, also you take no battle damage

Akki Drillmaster Akki Lavarunner Ankle Shanker Ardoz, Cobbler of War Battle Cry Goblin Blackcleave Goblin Boggart Ram-Gang Bolg's Company Broadside Bombardiers Cam and Farrik, Havoc Duo Chasm Guide Chimney Rabble Clamor Shaman Conduit Goblin Devoted Duelist Draconautics Engineer Fanatical Firebrand Fireblade Charger Flamestick Courier Frogtosser Banneret Fugitive Codebreaker Goatnapper Goblin Assault Team Goblin Berserker Goblin Bushwhacker Goblin Champion Goblin Chariot Goblin Chieftain Goblin Cruciverbalist Goblin Fire Fiend Goblin Guide Goblin Heelcutter Goblin Javelineer Goblin Motivator Goblin Rabblemaster Goblin Rimerunner Goblin Ringleader Goblin Ruinblaster Goblin Smuggler Goblin Striker Goblin Tomb Raider Goblin War Buggy Goblin Warchief Goblin-town Flunkies Goro-Goro and Satoru Goro-Goro, Disciple of Ryusei Grishnkh, Brash Instigator Grub's Command Hellraiser Goblin Hobgoblin, Mantled Marauder Howlsquad Heavy Hulking Bugbear Impulsive Pilferer Izzet Chemister Jeering Instigator Jund Hackblade Kiki-Jiki, Mirror Breaker Krenko, Baron of Tin Street Kyren Flamewright Kyren Legate Lavaborn Goblins Legion Loyalist Legion Warboss Mardu Scout Mizzix, Replica Rider Mudbrawler Cohort Oxidda Daredevil Party Crasher Prowcatcher Specialist Raging Goblin Raging Goblinoids Reckless Bushwhacker

But last year, Konami added multiple cards to the deck in the set Chaos Impact
Pairing well with everything from basic removal, mill, and even recursion-based strategies, Syr Konrad can suddenly accumulate large sums of damage seemingly out of nowhere