54 cards where (the text includes “gain life” and the text includes “opponent”)

Agent of Masks (Modern Masters 2017 #149)
Ajani, Strength of the Pride (Core Set 2020 #2)
Archfiend of Despair (Commander Masters #137)
Astarion, the Decadent (Commander Legends: Battle for Baldur's Gate #265)
Atarka's Command (Double Masters 2022 #176)
Blood Tithe (Magic 2011 #84)
Blood Tribute (Commander 2017 #100)
Bond of Discipline (War of the Spark #6)
Bubbling Cauldron (Jumpstart #460)
Chancellor of the Dross (New Phyrexia #54)
Cliffhaven Vampire (Oath of the Gatewatch #153)
Creeping Bloodsucker (Jumpstart 2022 #21)
Debt to the Deathless (Ravnica Remastered #176)
Defiant Bloodlord (Commander 2021 #142)
Dina, Soul Steeper (Strixhaven: School of Mages #178)
Epicure of Blood (Commander 2021 #143)
Erebos, God of the Dead (Theros #85)
Exsanguinate (Commander Masters #156)
Garruk, Apex Predator (Magic 2015 #210)
Gonti's Machinations (Aether Revolt #63)
Gray Merchant of Asphodel (Theros Beyond Death #99)
Gríma Wormtongue (The Lord of the Rings: Tales of Middle-earth #88)
Grim Feast (Mirage #265)
Hide // Seek (Dissension #151)
Jeskai Charm (Double Masters 2022 #232)
Knight of Dusk's Shadow (Dominaria United #96)
Kokusho, the Evening Star (Iconic Masters #95)
Malakir Bloodwitch (Crimson Vow Commander #131)
Marauding Blight-Priest (Zendikar Rising #112)
Ob Nixilis of the Black Oath (Commander Masters #176)
Oloro, Ageless Ascetic (Commander 2013 #203)
Plague Drone (Warhammer 40,000 Commander #47)
Planeswalker's Mirth (Planeshift #12)
Quakebringer (Kaldheim #145)
Roiling Vortex (Zendikar Rising #156)
Sanguine Bond (Commander 2021 #153)
Scholar of Athreos (Theros #28)
Servant of Tymaret (Born of the Gods #82)
Severed Strands (Double Masters 2022 #88)
Shadowgrange Archfiend (Crimson Vow Commander #22)
Shard of the Nightbringer (Warhammer 40,000 Commander #55)
Sorin, Solemn Visitor (Khans of Tarkir #202)
Subversion (Urza's Legacy #68)
Syr Ginger, the Meal Ender (Wilds of Eldraine #252)
Tainted Remedy (Magic Origins #120)
Talara's Bane (Eventide #47)
Tibalt, Rakish Instigator (War of the Spark #146)
Tormented Hero (Theros #108)
Tribute to Hunger (Duel Decks: Blessed vs. Cursed #65)
Valentin, Dean of the Vein // Lisette, Dean of the Root (Strixhaven: School of Mages #161)
Valentin, Dean of the Vein // Lisette, Dean of the Root (Strixhaven: School of Mages #161)
Vito, Thorn of the Dusk Rose (Core Set 2021 #127)
Vizkopa Guildmage (Iconic Masters #211)
Weed Strangle (Lorwyn #147)
Witch Hunt (Commander 2013 #133)

Though this query has ended, its relics still slumber in New Argive.