The same advice applies to free gaming as indie/proprietary.
If you've left it until your game is finished, you've left it too late! You should be doing marketing ever since you get a basic semi-playable alpha version, it allows you to obtain feedback and gain followers. Post dev logs, pretty gifs, and ask for feedback
Website - Create a website which acts as a primary "marketing" page for your game. It is
very important to include a good description, galleries, and links important information. It's also important to have a modern web design, as bad designs make potential players doubt your artistic skills
Create trailers - Create trailers and demonstrations showing gameplay. This is often the first thing a potential player will see.
Social media presence - Create Twitter/Mastodon accounts, etc. A lot of more casual free software users will use Twitter, and not mastodon. You can bridge the two. Post dev logs, lots of gifs, respond to questions, run polls.
Post on reddit and such - It's a good idea to contact subreddit moderators first. Good subreddits include
https://reddit.com/r/freegaming and
https://www.reddit.com/r/opensourcegames/