Why don't we use gerunds in these cases

Question

I understand en buvant. 
But why is this incorrect:
Sans me pressant (instead of presser)
Avant partant (instead of partir)
On passerait la journée discutant (instead of discuter)

Accepted Answer

Bonjour Suzanne,
‘pressant’, ‘partant’, ‘discutant’ are all present participles and not gerund (see link below)
The gerund (Gérondif) is usually introduced by ‘en’: ‘en + -ant’

en mangeant = while / by eating
en buvant = while / by drinking

‘Sans’ + present participle doesn’t exist in French. You use the infinitive instead:

Without listening = sans écouter
Without rushing = sans se presser
Before leaving = avant de partir

Passer [son temps / sa journée etc.] à + infinitive verb:

Il passe son temps à regarder la télé = he spends his time watching TV
Il passe sa journée à discuter au téléphone = he spends the day talking on the phone

Follow the link which will give you more details on the difference between the gerund and the present participle: le-participe-present-et-le-gerondif
 
I hope this is helpful.
Bonne journée !