Minecraft wiki entry for gamemode, associated text: Specifies the new game mode. Must be one of the following:
survival (can be abbreviated as s or 0 in Bedrock Edition) for survival mode
creative (can be abbreviated as c or 1 in Bedrock Edition) for creative mode
adventure (can be abbreviated as a or 2 in Bedrock Edition) for adventure mode
default (can be abbreviated as d or 5) for default gamemode ‌[Bedrock Edition only]
spectator‌[Java Edition only] or 6‌[upcoming: BE 1.18.30] for spectator mode
"hardcore" is not a valid option for the <mode> argument, as it is technically not a game mode.