The fix is really simple. Change TW Matchmaking to be based on Average GP signed up.
For example, a 240 GP with 36 VS 120 Gp with 45 wouldn't be in the same matchmaking band. The 240/36 guild would get paired with 300/45 guilds but prioritize matching as close to possible for equal active members. If a guild decides to sandbag by filling in spots with lower GP accounts, they lose out on Guild Raids, TB and other resource collection events and it wouldn't be worth. It handles Both types of sanding bagging with one solution.
Instead of taking into account the whole guild GP which would not be fair across all GPS because lower GP guilds rely on inactive players to qualify for TB events, which would hurt them in TW.