The city landscapes are based on eastern european countries, yes! ... Very likely they took inspiration from Belgrade, but I highly doubt it is all based on Belgrade.
HL2 is not set in eastern europe. They have just taken that art direction, one LIKE 'a suppressed eastern europe'.
So talking about guns not being available in different parts of the world makes no difference at all, theres also no place in eastern europe called city 17! :P (I think :O)
nice idea, but they are IMO a bit "tarty" like that. I think it could have been done better.. a little to much makeup for the theme of the game I'm thinking.
Mossman looks like she's been smacked up, Alyx's lips look plastic and Eli and Klieners faces look .... far to grainy (imo :x)
http://m00man.pwp.blueyonder.co.uk/hl2/d3_citadel_020001.jpg
http://m00man.pwp.blueyonder.co.uk/hl2/d3_citadel_050006.jpg
not the best screenshots but there ok.