How to display gamemode in Minecraft multiplayer server?

There is a very simple way of doing it with a plugin. Make sure you are running Spigot and download the PAPI plugin. Then move it to your plugins folder and restart/start your server.

Once the plugin is loaded, type /papi ecloud download Player. Then reload using /papi reload and use the placeholder %player_gamemode% anywhere you want.

Create a dummy variable: /scoreboard objectives add gamemode dummy

In a repeating command block, set their score to 1 if they're in creative: /scoreboard players set @a[gamemode=creative] gamemode 1 (You'll need to /forceload the chunk the repeating command block is in.)

And add it to the sidebar: /scoreboard objectives setdisplay sidebar gamemode

This will show in the sidebar whether someone has entered creative mode, even if they swap back to survival mode afterwards. To reset it, do /scoreboard players set PLAYERNAME gamemode 0, replacing PLAYERNAME with the name of the player.