If you want to change the gamemode just for you or for a short time, use the /gamemode command instead. The gamemode option defines which gamemode will be applied to players who never played on your server before. This setting does not affect the performance of your server. The slots define how many players can play on your server at the same time. If something does not work as intended, check your plugins first. Plugins can overwrite the behavior of some options.
In this guide, we'll describe to you all the available options for your server.