This isn't actually a rules question–as far as the game rules are concerned, both players can sit there happily +1ing their Gideons until one or both die of old age without issue. Instead, this is a matter of policy–how do we resolve a situation where players' actions are keeping the game state in a holding pattern that does not progress towards an end? As such, I've moved it to the Competitive REL forum.
Similar situations have been discussed before on the forums (most recently
here, if I remember correctly), though most of those discussions have tended to involve a single player continuing the holding pattern, with the other player completely unable to meaningfully affect the game state.
Personally, in this situation I would remind both players that they can concede or agree to a draw at any time, then ask them to make plays that advance the game state. If they instead continued the loop, I would issue Slow Play warnings to both of them and repeat my previous reminder and request. If they again continued the loop, they'd then both receive Game Losses for Slow Play, thereby ending the game.