I was going to buy some metal plate armor, but I want to know what to look for. I needed it so that I could do some test on it. I already have proper padding, so all that I need to know about would be the armor itself.
Is 16 gauge steel appropriate enough? How thick does it have to be? Is 5 mm too thick?

What weapons are you testing? What time period for the armour? What country? Do you want munitions armour or custom-made "knightly" armour? Do you have access to bloomery iron? What makes you think that the padding you have will be a decent replacement for what was worn in period? How are you going to determine whether a particular penetration is a disabling wound?

Read "The Knight and the Blast furnace". The book contains some of the most extensive testing on plate armour.

Sword thrusts against plate armor are pretty much guaranteed to not do anything.

Also 5mm is way too thick, if I remember correctly, plate armor wasn't usually thicker than 2mm.

Yep. You don't need any more tests to prove that. Williams demonstrated that an edged weapon requires 81 J to barely penetrate 1.5mm plate (p. 936). He also cites from Horsfall's study that shows that the hardest underarm stab that a person can deliver is 63J. Overarm the maximum was 115J.

In another test he shows that it requires 175J to fully penetrate 2mm mild steel with a spike. He then shows that increasing the carbon content by 0.25%-0.3% reduces penetration by a quarter. Increasing C by 0.5% halves penetration (p. 938).

14th century plate armour was worn over mail. Williams reckons that in order to compromise a 2mm coat of plates and a mail haubergeon one would need a total of 220J (p. 946).

On p. 947 Williams wrote that 15th century Milanese plate would be much lighter since mail was no longer worn underneath. A point striking 2mm plate at 30 deg would require 230J (280J with padding). Even striking normal to the target the required energy is around 190J without padding. No way can you do that with any kind of sword. Not even a warbow can do that.

Thanks for your answers, guys. Guess it's kinda pointless testing these Medieval Armors. But I still want to test the Roman Gladius and the Medieval pointy swords against some metal armor. What was the thickness, including the overlapping of plates, of the Lorica Segmentata? Is a 14 or 16 gauge steel a close representation of the metal quality of the Segmentata?