Is this gaming PC worth $400? I can’t tell if it’s too good of a deal...?

$400 for that? Yes, that's a really good price. The only thing holding it back is the 8gb of RAM part. Otherwise this PC is worth $500-650. The Core i7-8700 may be getting a little bit old but at least it has 6-cores with Hyperthreading.

Intel hasn't changed their architecture since the 6th Generation which came out in 2015. All they have done is added more cores and raised clock speeds. Supposedly next month, Intel is set to release their 11th Generation Rocket Lake processor which will finally be based on a new architecture but these will still be made on Intel's 14nm process. 

A PC with an older Core i7-4770 which is a quad core with Hyperthreading, 16gb of RAM, and a 6gb GTX 1060 is worth a little bit more than $400. In fact the value of this example PC might be a little bit higher than $400 because there is a shortage of Graphics cards.

I'd buy it and then add more RAM.

Yes, that gaming pc is worth $400 and it is not too good of a deal. In fact, that gaming pc worth $400 looks good for gaming and it is a good deal.

Seems a decent price for those specs.  You may want to upgrade the GPU at some point if you want 4K gaming now or in future but the basic specs are good for the price. A ram upgrade may be needed in future and I'd ask if there are free ram slots for easy upgrade.

A better PSU may be needed for certain gpu upgrades but you can factor that into any upgrade.

I think $400 is a reasonable price for it. 

Just note that it's an 8th gen CPU which is about 3 years old at this point.  But should still play almost anything you throw at it (Cyberpunk may make it cry, but anything else should be fine.)