Here's my honest grassroots assessment.
Most sinkies have not heard of GMS. To them, GMS is just one of those faces who show up once every five years during elections. If they know that GMS is now based in HK, and only flew back for the elections and will leave once he loses, they will further not support him.
The only sinkies who heard of GMS are those active in singapore cyberspace political forums. And most of them have a low opinion of GMS. To them, GMS is just a clown. And GMS proved that he is a clown by threatening to sue some facebook fella for defamation over some faked degree. The "costs" that GMS demanded was $150 to be donated to a couple of opposition parties. GMS behaved like an LKY, but falls flat on his face. The whole incident was nothing but a farce.
GMS is just some idiot with a big ego who thinks sinkies are looking up to him as a saviour. KJ is another idiot who is just milking his late father's memory for whatever opposition votes he can get.
The grassroots assessed that the only opposition that can pose a serious challenge is the WP. SDP, need to wait first and see, but generally SDP's name also not that good.
In our first-past-the-goal-post system, opposition votes used to be split amongst many opposition parties. After Cheng Bock lost by a whisker to KFC, people now will no longer let their opposition votes be split. Nearly all of them will now give it to the opposition party with the best chance of winning. You saw KJ and Desmond Lim lost their deposits in the PE by-elections, along with whatever shred of dignity they had.
The other opposition parties still haven't wake up yet. Other than WP, nearly all of them don't have any form of active grassroots. They got close to no chance of beating PAP at all even if all the MRT trains break down at the same time.