echelon99
Apparently both of these are correct in the GMAT but what are the rules for their usage?
I found one topic on this but with only one reply, and I'd rather get a conversation going!
I'm happy to help with this.

I would say ---
the verb "to plan", in all its forms, takes the infinitive.
I plan to go ....
She is planning to go ....
He planned to go .....
They had planned to go ....Use of the infinitive with any form of the verb "to plan" is perfectly correct and acceptable on GMAT SC.
By contrast, the structure "to plan" + "on" + [participle] is highly suspect. It is very colloquial. I would be inclined not to trust it: I can't imagine it would even appear on the GMAT.
She is planning on going to the store. --- Very informal and colloquial. Nothing this casual and informal appears on the GMAT SC.
For more on infinitive phrases, see this post:
https://magoosh.com/gmat/2012/infinitive ... -the-gmat/For more on participial phrases, see this post:
https://magoosh.com/gmat/2012/participle ... -the-gmat/Does all this make sense?
Mike