The only thing you'll need to know is the Label's name, if the name is 'Label1' then

var Temp: String;

Temp:= (Engine.Items.Find('Label1') as TGLXLabel).Caption;

you can also retrieve the pointer to the label:

var Label1: TGLXLabel;

Label1:=Engine.Items.Find('Label1') as TGLXLabel;
Temp:=Label1.Caption;

And the font issue. It'snt a bug in the font engine that the 512x512 font doesn't work, the sprite demo uses it and it works just fine for me (ATI Radeon 9700). Could be a driver issue or the graphic card that not supports that big textures.