A320 Hyd B+y Lo Pr Summary
 

In QRH 1.10 it is stated in APPR that L/G GRAVITY EXTENSION what is meant by that and Green system is not affected? and when do we actually read this summary is it following the ECAM actions.?

I would suggest that with both slats and flaps being driven from the green, the extra hydraulic load from operating the gear in the usual manner would be not a very good idea with blue and yellow out.

You read the summaries after completing the ECAM actions, starting with the cruise portion where you get the information to decide on which place to divert. After your approach briefing, read the landing and go around portions. Finally, before reading the approach checklist, read the approach summary. That sequence works well, you get the information you need in a logical way per phase of flight.
In B+Y Hyd lo pr, you lower the gear by gravity to preserve the Green hydraulic system integrity. Regards.