[offlinedoctor]

Is it right to assume that Gibbs Free energy is just the likelihood of whether a reaction can occur or not, while Entropy is just the tendency for reactions to go towards a more disordered state?

And, whats the difference between ΔG vs ΔGr?

[Corribus]

Gibbs energy is a chemical potential that describes whether a process will, given enough time, occur.  (And by "occur", it is meant that once equilibrium is reached, there will be more reactants than products, not that the process will transform all reactants to products.)  Gibbs energy says nothing about rate, however.  Entropy is one of the components that determines the Gibbs energy and has been colloquially described as a degree of disorder of a system.  It is a purely statistical concept.

[offlinedoctor]

Thanks for the reply, it makes a bit more sense.

This might be silly, but S vs S°, is the former done at different conditions while S° is done in standard conditions?

[Corribus]

The superscript denotes it is a reference value for a certain amount of the substance measured under a standardized condition.  The quantity without the subscript is the value for any other amount of the substance under any other condition.  By comparing the nonstandard thermodynamical value to the standard thermodynamical value (or the associated change during a reaction), you can then determine the spontaneity of the reaction.

This comparison is most often done with ΔG and ΔG°. 

[offlinedoctor]

Ahh that clarifies a lot, thank you!

Adding on from that, whats the difference between using
ΔG°= Sum of Products - Sum of Reactants
Versus
ΔG° = -RTIn(K)

I figured, if the reaction was at equilibrium, it would be possible to use both equations and get the same result. Is it because the former, formula, is using free energy not based on equilibriums?

[Corribus]

There is no difference.  You can use the standard Gibbs quantities of reactants and products (first equation) to determine K.  In fact, this is the usual way of doing things.