Can I use Gdevelop game engine?

Can I start my project now and make make a working UI , menu , character movement instead of waiting for jam to start  ? 

Um here's the FULL RULES that included in the jam itch.io page:

FULL RULES

• Make a game in the given time. The majority of the game must be made during the week. You may submit to multiple jams, so long as you don’t start before the start of this jam!
• The theme will be announced at the start of the jam. You are not required to follow the theme, it is only one category and is intended as creative inspiration.
• You may work alone or in teams.
• Windows or WebGL builds must be included, though supporting other platforms is encouraged.
• The game must be playable for English speakers. Supporting other languages is encouraged - this will be easiest if you use symbols instead of text.
• NSFW content MUST BE APPROPRIATELY LABELLED AND VISIBLE AT ALL TIMES. Yes, that means your game’s name and cover image should include a strong warning that it contains NSFW content, and the name and cover image MUST BE SFW.

if you didn't find anything that has your question answered then try text with the jam creators I guess

The main rule is that

The majority of the game must be made during the jam

You may use existing skeleton code - a menu and a character controller would be good examples of that. However, keep the above rule in mind - there will be space to declare stuff you've brought into the jam when you submit, make sure you use it so people can judge you fairly 😊