What a game design document is for
A GDD keeps everyone, including future you, building the same game. The one-page version answers the questions a publisher, a teammate or a game jam judge asks first: what is it, who is it for, what does the player do, why is it different and how does it make money. The full version adds mechanics, progression, art direction, the feature list and the plan. Write the one page first. If it is hard to fill in, the idea needs more thought before you write code.
Pillars and the core loop
Design pillars are three short promises, like "every death is fair" or "cozy, never stressful". When you are unsure about a feature, check it against the pillars and cut what does not serve them. The core loop is the few actions the player repeats every minute, such as explore, fight, loot, upgrade. If the loop is not fun on its own with placeholder art, more content will not save it.
The scope check
Most indie games run out of time, not ideas. Give each feature a size and a priority. The tool adds up the must-have features in person-weeks and compares them with your team, hours and months, keeping 30% for bugs, polish, store pages and surprises. If the must-haves do not fit, cut or shrink them now. Milestones are placed for you: prototype, vertical slice, alpha with every must-have in, beta and launch.
Is my idea uploaded?
No. Everything stays in your browser and is saved only on this device.