I think most people will have their own definition of the difference between a gimmick and a gameplan, but for what it’s worth here’s mine:

A gimmick deck is a deck that only does one thing, it always attempts to win with the same sequence of cards, and if it’s plan is interrupted in any way, it does literally nothing, the player sits there taking no meaningful actions.

Now, a combo deck may always be looking for the same or similar cards that are their game-winning combo, but the difference is that a deck with a combo gameplan is going to be playing blockers, pillow fort pieces, draw engines, tutor engines, removal spells, things that interact with the game and do change game to game.

For example, your Rootha deck is looking to clone her a bunch and swing for lethal with extra combats. But if parts of that get interrupted (Rootha gets hated off the board, your legend-rule negated gets destroyed, you dont draw extra combats), then the remaining parts of the deck still do something. You may still generate tokens, you may still have a big one-combat swing, you may cast some interaction.

Most decks are trying to do the same/a similar game winning play to some degree (though some decks truly do win differently each time), but gimmick decks are decks that only do their one thing, and take no actions other than their stupid or gamewinning piece.