QMK

Output for QMK, a firmware for custom keyboards. You can check my config here for reference.

1. Set up chords as per this gboards guide.
2. Add definitions for KC_CHORD, KC_CHORD_SFT, KC_CHORD_ALT1, KC_CHORD_ALT2 thumb keys to your keymap.c. Feel free to change the actions to whatever works for you. If you have other special keys on your letters (e.g. home-row mods), add definitions for those too so they can be referred to in the script. Use these in your keymap.
3. Move the #include "g/keymap_chord.h" line below all your definitions:

#define KC_SFT_A MT(MOD_LSFT, KC_A)
#define KC_ALT_S MT(MOD_LALT, KC_S)
#define KC_GUI_D MT(MOD_LGUI, KC_D)
#define KC_CTL_F MT(MOD_LCTL, KC_F)
#define KC_CTL_J MT(MOD_LCTL, KC_J)
#define KC_GUI_K MT(MOD_LGUI, KC_K)
#define KC_ALT_L MT(MOD_LALT, KC_L)
#define KC_SFT_SEMI MT(MOD_LSFT, KC_SEMI)

#define KC_CHORD_ALT1 LT(1, KC_TAB)
#define KC_CHORD_ALT2 LT(2, KC_SPC)
#define KC_CHORD_SFT MT(MOD_LSFT, KC_BSPC)
#define KC_CHORD C(KC_BSPC)

#include "g/keymap_chord.h"

1. Define the custom key codes in config.yaml:

output:
  qmk:
    key_codes:
      A: KC_SFT_A
      S: KC_ALT_S
      D: KC_GUI_D
      F: KC_CTL_F
      J: KC_CTL_J
      K: KC_GUI_K
      L: KC_ALT_L
      ;: KC_SFT_SEMI

1. Copy the generated ~/.config/chordgen/qmk_chords.def to your QMK keymap directory and add #include "qmk_chords.def" to the top of your QMK chords.def file.
2. Flash your keyboard.