Do the A-320 landing gear extend if the mechanism is activated above a limiting speed?
The hydraulic pressure of the landing gear is shut off above 260 KIAS. The maximum gear retraction speed as for the placard in the cockpit is 210 and the max extension speed is 250 KIAS if I'm not mistaken.
From the FCOM 1.32.10
(download from here for example: http://www.smartcockpit.com/plane/AIRBUS/A320.html)
A320 Landing Gear, page 3
OPERATION OF GEAR AND DOORS

NORMAL OPERATION
The green hydraulic system actuates all gear and doors. When the aircraft is flying faster than 260 kt, a safety valve automatically cuts off hydraulic support to the landing gear system. Below 260 kt, the hydraulic supply remains cut off as long as the landing gear lever is up.

The logic is then described in a diagram in the FCOM (see link above).

If ( ADR1 &lt; 260KT || ADR3 &lt; 260KT || MLG on gnd ) &amp;&amp; ( LG lever down || safety valve already open )
-&gt; open safety valve

If I interpret this diagram correctly you can leave the gear lever down and the gear should start to extend once below 260 kt because the second side of the and gate just requires the gear lever to be down.
On the contrary if you want to retract the gear and you fly faster than 260 kt then the safety valve closes. Even if you fly slower than 260 kt it will stay closed because the second half of the and gate is not fulfilled. You need to move the gear lever down, then the safety valve opens and then you can move it back up and the self maintained state of &quot;valve already open&quot; will allow you to retract the gear after this cycle of the landing gear lever.
Part 2 of the questions regarding the indications
Indications are as follows:

3 red lights above the gear lever as long as landing gear is not locked in the selected (down) position according to A320 FCOM LANDING GEAR page 18, GEARS AND DOORS, LANDING GEAR INDICATOR PANEL
A red down arrow next to the landing gear lever when the gear is not locked down when the aircraft is in the landing configuration (page 19)
6 red triangles on the ECAM wheel page (3 gears, two redundant landing gear control units) (page 21)
An amber pulsating L/G CTL legend on the ECAM wheels page when the landing gear position and gear position do not agree (page 22)
GPWS audio call out and associated red light outboard of each PFD (see below)
ECAM warnings as follows

ECAM warnings according to the WARNINGS AND CAUTIONS section of the FCOM LANDING GEAR chapter, page 23:


red flashing master warning and cricket aural warning: GEAR NOT DOWNLOCKED on the upper ECAM display, ECAM WHEELs page is called automatically on the lower ECAM display with the indications stated above
red flashing master warning and cricket aural warning: GEAR NOT DOWN, when below 750 ft radar altitude with both engines below 75% N1. Other conditions require flaps to be extended to 3 or FULL but above 260 kt you probably would cause structural damage if you were to attempt that.


From the A320 FCOM Chapter 34 NAVIGATION, GPWS, page 6.

Two aural warnings may be triggered, depending on the area: &quot;TOO LOW-GEAR&quot; [below 190kt and 500ft RA] or &quot;TOO LOW-TERRAIN&quot; [above 190 kt, up to 1000ft RA]

If you are flying faster than 260 kt the GPWS should create a &quot;TOO LOW-TERRAIN&quot; warning.