It is airline SOP specific.
I have used a couple of SOP's and in both cases the gear is selected at a specific point in relation to glideslope intercept; namely a dot and a half fly-up deflection.
You will notice that at most airports when you look back up the approach at the approaching aircraft, that they may appear levelled-off for some distance before beginning their descent down the glideslope. This is known as the level-off or platform altitude. As the aircraft approaches the glideslop signal, flying level, the fly-up deflection will get less and less; upon reaching one and a half dots (two being maximum) the gear is usually selected.
The gear can be selected earlier if determined necessary and safe by the flight crew. This may be required on 'hot' approach where the aircraft is both high and fast and requires more drag.
Or the gear maybe selected later where a high speed approach is required e.g. 160 knots to 4 dme, and selecting gear in the normal position would just result in more power being required uneccessarily.
I haven't explained that very well
Can anyone else help?
RBG