[GPhab]

If  :delta:G of a reaction is negative, does it mean that the reaction WILL occur and that too spontaneously or that the reaction may occur and if it does, it does so spontaneously?

[Astrokel]

hey, the reaction will occur spontaneously, without any external triggers(input of energy into the system), however the spontaneity does not mean the reaction will happen immediately or fast.

[Yggdrasil]

hey, the reaction will occur spontaneously, without any external triggers(input of energy into the system), however the spontaneity does not mean the reaction will happen immediately or fast.

Basically correct, but I would say that the reaction will proceed with no NET input of work into the system.  Most reactions will require some input of energy to overcome the activation energy associated with the reaction.  For example, burning gasoline is thermodynamically spontaneous but it requires an energy input in the form of a spark to proceed.  However, the process overall requires no input of energy to sustain itself once it starts going.