What you have are two different kits, maybe even three.
First of all, a gambeson of that length is often worn with a hauberk, placing it in the 12th to 14th century (no aketons or gambesons pre-12th far as I know). With this, you'll also want a conical helmet with a nasal and a chainmail coif. A large shield and a long spear with a nice 11th/12th century sword at your side.
However, the incredible thickness of your gambeson actually places it in the "Standalone cloth armour" area, making it a commoner-armour for the 12th century up to about 1330/1350, meaning that adding a kettle helm to it and either a dane-axe (normand), spear and shield (12th through 13th) or a polearm (early-mid 14th century).
Now, for plate, and in particular late 14th and early 15th century, you'll not only want a shorter gambeson, but in fact, something way, way, WAY thinner. A slightly padded jacket (aketon, pourpoint, arming jack etc.) is more than enough because you cover everything else in either plate or maille voiders. Furthermore, with a gambeson that thick, you'll actually run into problems bending your arms in plate gear. Skallagrim made a review of his Armstreet armour and ran into this exact problem (mistakenly blaming the armour and not realising how monstrously thick his gambeson was).
As for "long gambeson with plate", there are several reasons for not having one:
First of all, it's extra weight, plenty of it in fact, and that's just straight up bad.
Second, it's extra material in cost.
Third, you actually cover your groin with hanging maille that may or may not be padded (most likely not) and there simply isn't a need for cloth hanging that far down.
Fourth, your thighs will be covered in plate and the gaps in maille, there is no need to heap lots of cloth on top of it.
Fifth, it's incredibly old fashioned. At best you'll see some cotehardies of knee-length around the 1360's and 70's, but they are much rarer than later pieces.
Sixth, it's actually a bit of a risk, since it can get in the way. It will also restrict your movements slightly more than the well fitting plate legs underneath do.
I know it's cool. And I know you want to do it, but if you're asking "did they?" or "were there any chance?" then I'll say no. Not 100% absolutely mega-no, you can never do that, however, there is no historical basis that a transitional plate harness of the 1380's to 1410's would ever wear any sort of arming garment reaching down across their thighs.
As said, your gambeson is simply too thick to be worn with plate and too long to be worn after the 1350's.
However.
There are some depictions of people in 1340's and 1350's coats of plate and similar that DOES have longer aketons underneath their coats of plate. But, these are likely thinner than what you have, and those are still far away from the armour you're wearing.
So, if you want to be historical, either get a new gambeson, or swap your breastplate for a well made coat of plate, or drop the plate, add an earlier period helmet and supplement with a large shield, a spear, a nice sword and either wear a hauberk on top or go to war in your very thick gambeson.
However, if you don't want to be historical, go ahead, it's a cool look and if you like it, you like it.
