If Thanos is in character, it's plausible that the WG could kill Thanos I guess.
Because WG powers don't protect against anything here (Neither Thanos nor his Infinity Gauntlet, with/without the stones are divine/godly), Thanos can pull off anything he wants w/o it being blocked by WG power. IE, toss the WG into the sun, time freeze (QBD does this on the player, WG or no), etc. Basically, Thanos could easily kill the WG, but if the movie is anything to go by, he wouldn't do so, not unless it's life threatening.
Till then, it's a physical brawl using the stone powers. Deflect Magic does nothing (possibly could help vs a pure power attack?), but Deflect Range/Melee could help even the odds - considering both regular Jad and Obsidian Jad (they can OHKO players, but Protection/Deflection prayers heavily negate their attacks AND reflect some damage back). Prayer flicking between SS and Deflect curses while using T99 leech curses could actually make this a thing.
Inb4 Runescape pulls a Fortnite and throws in Thanos as a ingame boss
That being said, each stone individually could easily **** over the WG: Power Stone OHKOs by sheer power (this is a thing ingame too, bosses can do it all the time), Space stone can BFR (player can counter this by teleporting back, though their teleports are limited to set locations, they can teleblock Thanos and very theoretically themselves), Reality stone could negate the WG's equipment, Soul stone could counter Soul Split & possibly OHKO WG if the page info is accurate, Time Stone does time stuff (WG has NO COUNTER to time manipulation, as seen with Queen Black Dragon's timefreeze attack as well as the Celestial Dragon's special effects)
But of course, all of that means nothing, nothing at all...
DEATHTOUCHED DART INSTAKILL FTW BOYS, DOING WHAT THOR CAN'T DO