Can I use mr muscle drain gel in my toilet ?

Doubt it will work because that is for dissolving fat/grease deposits whereas you have different solid matter to shift. I use a flexible wire drain rod that goes around the bend to clear any drain blockages these are cheap to buy and can be bought in most DIY stores - if you cannot get one just cut and open-up an old wire coat hanger or hard wire cable and use that to break the blockage into tiny bits that will flush.

You need to establish where the toilet is blocked, is it just the toilet or the drain beyond? Mr Muscle won't hurt the toilet but it's unlitely to unblock it. If it's the drains, they will need to be cleared with drain rods.

put a bit of bleach down and use the toilet a few times before flushing, until you need to go again only flush after you have been then hopefully it will go.