Ethernet and networking speeds as "500 Mbps Gigabit"?

From the TP-Link product spec:
It has a Gigabit Ethernet Port - that's where you connect your your PC/laptop/whatever device) and it can transmit 500Mbps over powerline.

As for 10/100 and the rest - as you said, it's related to Ethernet standards, but in simple terms just tell you 'speed' a device can operate:
10/100 mean either 10Mbps or 100Mbps
10/100/1000 - 10Mbps or 100Mbps or 1000Mbps etc.
Now, when you connect two devices, they will operate at highest common speed (one is 100, one is 1000 -> operate at 100). If they do not support such (say one is set to only allow 10Mbps and other only 1000Mbps) they will fail to communicate.

Going back to your question - it's similar logic here: 1G/500M plug will limit you to max 500M, but 100M/500M (non Gigabit one) will be max 100M as that's lowest speed along the transmission path. If the price difference is small it makes sense to go for Gigabit one - unless you know that none of your devices will ever support Gigabit.

Edit: I have not used those devices, but just googled a bit and it looks those high speeds (over power) are a bit (cough) overinflated. So if you have a chance it would be best to actually try both and see what actual speed you get!

It's a bit cheeky describing it as Gigabit but I think the gist of it is that the network port runs at 1Gbps but it can only send half that over the power lines. The one that doesn't mention gigabit only has a fast ethernet port for 100mbps so you wouldn't get the performance of 500mbps unless you were using more adapters. I think the 500mpbs is split amongst all the powerline adapters you have.

Each of the standard Ethernet speeds, 10Mbit/sec, 100Mbit/sec (a.k.a. "FastEthernet"), and 1000Mbit/sec (a.k.a. "Gigabit Ethernet") have different signaling and negotiation methods.

So maybe this adapter supports the Gigabit Ethernet signaling standard but just can't do the full 1000Mbit/sec over the link layer.

Guess it's possible e.g. a FastEthernet interface to actually operate faster than 100Mbit/sec but still use FastEthernet signaling/negotiation. However, a Gigabit interface connected to it would negotiate down to FastEthernet, and still transmit at that maximum speed, so I don't know what is going on with that. It's entirely possible the specs are incorrect or written by non-knowledgeable people.

One advantage Gigabit Ethernet has even if you aren't getting a full 1000Mbit/sec is "jumbo frames" which increase LAN (not Internet, of course) speed by allowing bigger Ethernet frames (8000bytes instead of 1500bytes), which can mean a higher MTU, which means reduced packet header overhead.

The specification states that the device supports IEEE802.3ab, which means that the Ethernet ports do support Gigabit Ethernet. The powerline transceivers, however, have a maximum rating of 500 Mbps in each direction (assuming full-duplex).

The actual total throughput will depend on a lot of factors, such as:

• length of electrical cabling between powerline devices
• whether or not the powerline devices are on different circuits, and if there are any circuit breakers between the devices
• age of electrical cabling
• interference/noise from other electrical equipment
• the speed of other networking equipment

Note that the so-called "200 Mbps" powerline devices often have 100 Mbps Ethernet ports, and are therefore limited to 100 Mbps in each direction (which could sneakily be called 200 Mbps, in half-duplex). This would also be the case for the non-Gigabit version you refer to.