I can not play on GG Poker due to my location. But as I understand it, the second phase is just like a normal MTT, so it does not make sense to talk about a special strategy. Player pool tendencies will depend more on, which stake you play, than this particular format. So just play like in any other MTT.
As I understand it, the first phase is an all-in shootout with the twist, that each player is dealt three cards, everyone can see all the cards, and then each player has to pick one card to discard. To play this phase well you probably want to spend some hours playing around with a
equity calculator like Equilab to see, how different hands tend to do in multiway pots.
In general the value of suited and connected hands go up multiway, whereas the value of high cards go down, since making one pair will often not be enough to beat up to 8 opponents. You also want to avoid being dominated or drawing to outs, that dont exist, because they were folded. Say someone else have KK2 and you have K75 rainbow. Then continuing with 75 will almost always give you a better chance of winning than continuing with K7 or K5.