Hi Steven
I've seen the Gazelle tyres on my fathers Guzzi Galetto and they are a bit soft and flexy looking. Good for this type of bike or other light weights with limited sporting potential, but not really suited for a 250/350/450 bevel in my opinion.
Have you tried Heidenau? they make tyres in the right sort of sizes and in race compounds as well should you need it. They have been used in the CRMC on various bikes to good effect. Not expensive and would my first port of call for thin tyres, with good rubber.
http://www.reifenwerk-heidenau.de/?lang=enM&P in the UK seem to stock them, but I am sure there are other suppliers.
Regards Nigel