I never find much difference in use of infinitive and gerund. However, it is always suggested that few verbs are always followed by gerund whereas few are followed by infinitives only.

I found a good distinction somewhere over the Internet which may help

Gerund

Often we use the gerund for an action that happens before or at the same time as the action of the main verb.

I enjoy playing (I enjoy myself at the time of playing.)
I denied stealing (I deny having stolen anything before.)

Infinitive
Often we use the infinitive for actions that follow the action of the main verb.

I decided to visit my uncle (Visiting my uncle was an action of my decision. It comes after.)
I want to go out. (What I want (now) is to go out (after/later))

In your example:

Someone suggested going for a walk.

He suggested for a walk at the same time to be happened not sometime in future.

However someone may suggest for future action too;

Someone suggested to meet Ms Rekha before leaving the Mumbai.

So I personally don’t consider that sentence as wrong sentence until the context is given. I found a good explanation for above situation here;