The pipe is the outlet for sink, dishwasher, washing machine. I have seen 92.5 degree bends so assume I would us this if I was to drill with a fall.
Also is there a minimum height/length for the outside pipe from the hole I drill down to the bottle gully?
If you use DWV fittings to transition from vertical to nominal horizontal the fitting will provide the right slope. Rather than try to drill to within about 2 degrees of true horizontal (and you can't use your wall as a reference for true vertical, you have to use a level) .... just drill 1/16 inch bigger than you need so you'll have enough play for the fitting to do its job. You'll have to reference the pipes to a spirit level but not the hole.
All drain pipes need a minimum slope. For typical household drain pipe sizes that's 1/4" per foot, or roughly 2% grade (so 2cm/100cm.)
If you can drill to allow a slight fall that would be better than a level hole through the wall.
As others stated a 1/4" drop per foot is preferred.
