Since this is a free product, I'm not doing an extensive review on it. You can just go pick it up and read it. I'm going to tell you why you might want to, and why you might not.
Challenge of the Frog Idol
23 pages (21 counting the cover but not the OGL license pages)
What is it? It's a basic hex-crawl, sandbox adventure for just about any old-school D&D clone. What does that mean? You have a base of operations and a mapped-out area to explore. The adventure is non-linear, so you pretty much can explore the swamp as you will. It's a location with lots of sub locations instead of an "adventure" or "dungeon" per se.
How does it work? Pretty well.
The town is sparsely detailed but it's enough to use it as a base of operations. It's got enough issues and conflicts to affect adventures and provide springboards to more conflict (aka adventures).
The swamp is much more detailed, with encounter areas well set up to push or pull adventurers in the direction of still-more encounters. The random encounters are interesting, the monsters appropriate, and the set-piece locations are pretty cool. They have enough detail to run them off the top of your head, but not so much that'll you need to flip around finding out the "right" way to run them.
Why might I not like this? If you hate swamps, you love railroad or linear adventures, or you want very detailed (instead of sparsely detailed but evocative) locations, you might not like this so much. You could stick the adventure on rails, but it's not designed to be so.
How does this work for GURPS? You'll need to convert stuff, if you want to run it straight-up. But with some lizard men, giant frogs, and plain-old swarms, you'll be fine running it just porting over critters as needed. You could easily change any given D&Dish Monster X into GURPS Monster Y without a problem.
Content: 5 out of 5. Excellent quality and it includes everything you need to run it.
Presentation: 5 out of 5. Attractive and easy to use.
Overall: If you need to fill in that as-yet-undetailed-swamp in your game, grab this and drop it in. Recommended.