There is a YouTube channel by the name wifi study channel. Go to that channel and watch videos on general math. It's an Indian channel. Videos of a person named sahil, are available on math. It will cover your whole ability portion of GSA. The guy has a really good method of teaching. It helped me a lot.
Just make sure you go in the right section of this channel, that is RRB GROUP D 2018, math by Sahil sir. Because he also teaches advance math for some engineering exams on that channel.
He uses a board, explains every mathematical step of a question and explains it well. Also, you need to start watching from the very 1st video. Videos are numbered and go on systematically. If you miss even one class/video of the basics, you won't get the next ones.
P.s i don't know how to start a new thread. If someone here does, please post this in a separate thread to help all the aspirants. I know that a hell lot of aspirants need help with this portion and there are no online classes, groups available on fb or anywhere else for this portion.