Gumtree is one of the most popular local options for selling off unwanted stuff, in part because basic listings don't cost anything. However, bear in mind one restriction: you can only edit your ad up to 12 hours after posting. If you want to make changes after that, you'll have to pay a fee.
Gumtree outlines the policy in its help documents. Editing is free for the first 12 hours (and you can see how much time you have left before editing closes by hovering over the question mark in the ad). After that time expires, you need to bump it to a featured advertisement to edit it, which will cost you $2 (and possibly more if you add other extras). As this Whirlpool thread points out, it's quite easy to copy the text from your existing ad, delete it and then repost it rather than editing it and paying for the privilege.
For what it's worth, I don't think this is an unreasonable policy. Gumtree is expected to make money somehow (it's a subsidiary of eBay, so it doesn't exist just for the good of the community). If you write your ad carefully before posting, you should be able to dodge the fee.