The level gap problem manifests as such:
1.Players with very different levels or skills want to player together
2.The area requires levels/skills of quality only achieved by one or few players in a group.
3.The higher-level/Higher-skilled player assumes the burden of the party.
4.The other players appear to be useless and just tag along for extra exp/loot.
1.Level-scaling(adjust monsters to average player levels): least realistic and nullifies level/skill investment since everything is easy, but simplifies the below solutions.
2,Reduced exp gain from high-level monsters for low-level players. Mainly serves to reduce "rushing" tactics where high-level players help level up others.
3.Loot quality dependent on average level of players. Usually implemented as picking loot suitable for low-level players instead of expect high-level loot.
4.Non-level progression systems: skill levels or item-based.