Gazelle? Does anyone know anything about them?

See above.
It's posible that it's a very old Gazelle. I don't know these frames so well.
Or another frame with a Gazelle head badge.
So bad, when people repaint such a nice frame, because you loose all information about it.

Also the rear dropout and the finishing of the chain stays look a bit strange.
In combination with the falling top tube: maybe an old road frame with new, track dropouts?

I'm pretty sure your last guess is correct. The frame looks identical to an old Gazelle 'Sprinter Race'. As far as I can tell it's a semi-race/touring cycle at the lower end of Gazelle '60s/'70s output (although it seems to have 531 plain gauge tubes). They changed the dropouts and put on a fork from another frame, resulting in the lowering of the front.

Here's some pictures of an original one: http://www.retrobike.co.uk/forum/viewtopic.php?f=47&t=241845