Support for Synaptics clickpads (Lenovo ThinkPad E540): missing soft buttons -- no corner tap action

[sphakka]

With sys-libs/gpm-1.20.7-r3 (Funtoo ebuild), I can use the clickpad on a Lenovo ThinkPad E540 (kernel support is RMI4) with types ps2, imps2, exps2: apparently all work the same, but there's no "soft" button support, only the hard (click) button and tapping work.
Indeed, this kind of touchpad, identified as follows...

52: PS/2 00.0: 10500 PS/2 Mouse
  [Created at input.249]
  Unique ID: AH6Q.KF1c0xgbUt4
  Hardware Class: mouse
  Model: "Synaptics TM2722-001"
  Vendor: 0x06cb 
  Device: "Synaptics TM2722-001"
  Compatible to: int 0x0210 0x0001
  Device File: /dev/input/mice (/dev/input/mouse0)
  Device Files: /dev/input/mice, /dev/input/mouse0, /dev/input/event14
  Device Number: char 13:63 (char 13:32)
  Driver Info #0:
    Buttons: 1
    Wheels: 0
    XFree86 Protocol: explorerps/2
    GPM Protocol: exps2
  Config Status: cfg=new, avail=yes, need=no, active=unknown

...has 3 soft buttons, configurable under Xorg/libinput on the top and bottom edges. How to do the same with GPM? It looks like the Synaptics-specific conf file /etc/gpm/gpm-syn.conf is ignored: all the pad area is available to the pointer and for tapping, but no "corner tap action" is enabled...

With -t synps2, GPM shows only a fixed cursor. Cf. debug logs gpm-synps2.log and gpm-exps2.log