[cycomanic]

Actually, if I'm not wrong your example 2a and 2b is about simple present vs present continuous not gerund. Isn't gerund something like "I like swimming" or "I like to swim" (where both works) or while "I enjoy swimming" only works with gerund?

Admittedly both are subtle and somewhat confusing to non-native speakers.

[HelloNurse]

Present continuous is built from gerund, like other continuous tenses. The important difference is therefore between the "simple" verb and gerund. What you refer to is a different use case of gerund.