The strategy behind Plox/GG is to get a Gardevoir Secret Wonders into play, load it up with energy, and then repeatedly shut down the other decks in the metagame with its "Psychic Lock" attack (most decks relied heavily on Poke-powers back then, which are all shut off by Psychic Lock). Ideally, you would would swarm with other Gardevoirs, and then occasionally use Gallade to deal extremely significant damage to the opponent's Pokemon (read the card and you'll see just how brutal it was...Think Reshiram and Azelf rolled into one).
In the Holon Phantoms-on format, your typical Gardevoir build looked like this...
2 Baltoy GE
2 Claydol GE
4 Ralts SW
2-3 Kirlia SW
3 Gardevoir SW
1-2 Gallade SW
1 Chatot MD
1-0-1 stage two tech line (Dusknoir became the most commonly-used line, since it added yet another layer of major board control on top of power lock)
0-4 basic tech cards (Absol ex, Pachirisu GE, Jirachi ex, and Jolteon* became the three most commonly used cards for these spots - each serving a different purpose)
Trainers/Stadiums/Supporters:
0-3 Windstorm
1-2 Lake Boundary
4-6 mix of Celio's Network and Bebe's Search
4 Rare Candy
4 Roseanne's Research
2-4 Team Galactic's Wager (used to make the lock even more devastating)
Energy:
4 Call
4 Double Rainbow
2-4 Scramble
3-7 mixture of Psychic, Fighting, Multi, and Holon Energy WP (the fourth of which was reprinted and thus legal for play)
Although there's a lot of variation in this list, it should cover pretty much every style of Gardevoir/Gallade/Claydol played back then.
Later on, when DRE and Scramble rotated, Garde/Gallade became "defanged," and thus needed a reboot. So for about a year and half, it was a mediocre, upper tier three deck.
Then, when Double Colorless Energy and Spiritomb AR came out, its viability increased exponentially, since it: A) regained its energy speed; and B) regained a chance at early game disruption via 'Tomb. The build of this 2009-2010 era looked much like what you see above from 2007-2008, only plus some modified legal choices (Uxie) and minus the cards that rotated out.