This question would probably better suited for the Project Management Stack.
As was stated GTD is a methodology for processing and gather your to-do items.
In the context of a software development project, what you are trying to create is a backlog of items to be completed, which are related to the overall product. A project, in Project Management terms, must have a defined beginning and end; therefore, if the "project" doesn't actually have a deadline, you are not actually working on a project - by definition. But, you can still work on the product - most likely using something like a Lean Software development approach.
Capture all the ideas you have. Review the ideas. Prioritize the ideas so you work on the most valuable items first (how you determine value will be based on your individual product). For example, you may have an item that say, "create storyline" - can't have a game without a story...in most cases. You may have another that says, create character models...and so on. All of these items are essentially marked with an "as soon as possible" due date. Then, just work on each item as you have time - constantly adding to your backlog as new ideas come to you; or, new things to consider come up.
In conclusion, it is totally possible to work on a "project" as defined in GTD terms (related to-do items with a defined goal/objective); however, in Project Management terms, it is not a project. A standard software development methodology for such an effort is Lean Software Development and/or Kanban setup.