As is your wont, you are essentially correct in your critique of common usage on both sides of the Atlantic.![]()
'Guy', as a first name for males, is much more common in the UK and in continental Europe than in the USA, so the risk of confusion between the particular and the general is greater in the former jurisdictions - I don't know if this is a significant factor, but it may be. I can also surmise at the origin of the corruption of 'girl' into 'gal' over a period of time. Upper class ladies of a certain age tend to pronounce the word 'girl' as 'gel' (with a hard 'g') in the UK and in the erstwhile colonies - it doesn't take a huge leap of the imagination to see that term further evolving into 'gal'.
The common terms for fellow males in the UK are much dependent upon geographical location, and even more so upon socio-economic standing. Up North it would be common parlance to "Have a jar with the lads," at your local pub. They might also use 'mates' further South, but that would imply friendship (which 'the lads' does not automatically). Australian English is more influenced by American English than is the case in the UK, so I have heard younger Australians refer to 'guys' as a generic term for their fellow males. But it is not a form of address in Australia (so no "You guys!"), and is never used with reference to females.
"Chaps" is now a somewhat outmoded descriptive term, but still in use amongst the middle and upper-class Public-School educated. 'Bloke' is common to the working classes in both Australia and the UK. "Lad" as a term (often of endearment) for a youth or boy, is common across the class spectrum. Other terms such as 'fellow', 'cove', or 'bod' are usually confined to the use of sexuagenarians, and, in my experience, 'dude' is strenuously avoided by all but the most committed and juvenile Americanophiles.
(Why is the spell-check chucking a wobbly over both sexuagenarians, and Americanophiles?)![]()