The effect of Shield Wall is: you become high cover.
That's it. You don't get any sort of defense bonus, you don't get damage resistance, nothing else. On the bright side, it doesn't appear to prevent you from taking cover, either. The net effect is that you're a regular soldier in every way, except that wherever you're standing happens to count as a high cover tile.
EMP Grenades/Bombs
So I just learned from Are Gatekeepers vulnerable to EMP Bombs? that Gatekeepers can actually be heavily damaged from EMP grenades/bombs, and can even have a chance to be stunned by them for 2 turns!
I tested and confirmed this myself. In the following screenshot, I used an EMP Bomb on an undamaged Gatekeeper, and it took something like 12 damage, and was also stunned:
Which is weird, because you can't hack a Gatekeeper, so who knew it had robotic components that could be damaged by an EMP?
Flashbangs
I've heard that Flashbangs (or being disoriented) can lower a target's Will stat, which would make it more vulnerable to mind control. I decided to test this out too.
The following is the Will stat of an undamaged Gatekeeper versus a fully upgraded (i.e. has all abilities and fully upgraded Alien Psi Amplifier) Psi Operative using Dominate:
The target's Will is 110%, which reduces my chance to Dominate to 80%. After hitting the Gatekeeper with an EMP Bomb, followed by a Flashbang (so that it's also Disoriented), I checked the chances again, and the Gatekeeper's Will was still 110%.
I've also heard that Flashbangs (and being disoriented) can disable some of the Gatekeeper's abilities, but I haven't tested this out yet, and I'm not sure which abilities exactly would be disabled.
Best Answer
Yes, all skills work regardless of tech level. "Vektor Rifle" is the name of the weapon class, as well as the name of the tier-1 weapon in that class. It's a little confusing.