En entrant, j'ai vu Marie: one subject, and tout en

One subject only

The gérondif belongs to the subject of the main verb. The person eating is the person watching:

• En entrant dans la pièce, j'ai vu Marie. - As I came into the room, I saw Marie.

So the gérondif cannot say I saw Marie coming in with Marie doing the coming: en entrant would mean that I came in. Use an infinitive or qui:

Fixed sayings bend the rule (L'appétit vient en mangeant. - Appetite comes with eating), but in your own sentences keep one subject.

Tout en: while, and even so

Put tout in front of the gérondif and the two actions pull against each other: even while, although.

• Tout en souriant, elle a dit non. - Even as she smiled, she said no.
• Tout en comprenant tes raisons, je ne suis pas d'accord. - Although I understand your reasons, I do not agree.
• Il a réussi tout en travaillant très peu. - He passed even though he worked very little.