Gandalf fights the Balrog in different TTRPG systems.

5th edition: Gandalf casts hold monster, the Balrog uses a legendary action to automatically make the save and then kills him

Pathfinder: The Balrog has spell resistance and teleport spells, there is no escape. Gandalf dies

PbtA: "You shall not pass!". When you stand firm and refuse to let your enemies get past you roll+Valor
On 10+ you and your allies may escape
On a 7-9 your allies may escape but you must hold the line.
Gandalf rolled a six.

Forged in the Dark: standing before the Balrog is a Desperate action with Limited effect. Gandalf pushes himself for increased effect. The GM offers a devil's bargain " you will be separated from the fellowship". Gandalf gets a 4/5.

OSR: Gandalf casts hold monster and it miraculously works. The DM rolls and the bridge breaks beneath them. Gandalfs player rolls on the "when a wizard dies table" to see what happens and gets the "reborn anew" result.

4th edition: Gandalf uses a daily power "Flame of Anor" from his epic destiny "Servant of the secret fire" to blind, stun, and immobilize one creature and uses an encounter power to break the bridge. The Balrog uses "whip tether reaction" to pull gandalf to an adjacent square

Shadowrun: Gandalf rolls a power 5 hold spell using channeling+willpower opposed by the balrogs willpower+magic resistance with a net four hits. Then he rolls a power 5 shatter spell versus the bridges hardness plus the skill of the engineer who built the bridge minus dice for...

Genesis: Gandalf uses his capstone ability "you shall not pass" with its potency upgrade for "nemesis" enemies to prevent him or the Balrog for taking any actions for three turns. He rolls a despair and a triumph.