As someone who travels in Europe a fair bit I feel your pain.
1 km = .6 miles
1 liter = ~.25 gallon (1 quart)
F to C (rough) subtract 30, divide by 2 (60F comes out as 15C, it's really 15.5)
C to F - double and add 30
These will at least keep you in the neighborhood and I find they're pretty easy to remember.
For sizes, afraid I can't help there, except most shoes have both listed on the tongue.
As an European who is reading US stuff a lot, I find it very easy to deal with different units:
Miles to kilometers:
Our hikes in the Alps are so much more tedious and difficult, that you are not too far off to think of 1 km equals to 1 mile.
On a really tough hiking day I might cover 20km horizontal, so I belive that I could hike a 20 miles per day pace in your part of the world.
Liter to gallon:
US hikers are educated to drink so much more water than we do, so while you might calculate a gallon a day, we calculate a liter per day.
Thanks for the rough conversion of temperatures, I was missing this so far.
Regarding sizes, we hardly buy new clothes or shoes, just use old stuff for the outdoors.
To the original topic, If I were buying new gear now, I would not buy again:
Ultralight stuff like the Exped UL sleeping bag I'm using now.
It works, but is so terrible flimsy, made of such thin and fragile material, that I'm constantly afraid that it will suddenly break.
And, its cut very narrow and with a short zip only, so I can slip in only just-so, and lie straight on the back and not move around much at all.
Next buy, I will go for the "Light" version, not the "Ultralight".
I consider everything I bought when I started part of the learning process, but just to give the short answer to your question about 95% of my original gear. I started off with an Osprey Crescent 90 pack, some huge super heavy Thermarest pads, a large overrated synthetic bag, and a bunch of other unnecessary crap.
AT: 2007-2019 (45 sections)
JMT: 2013