Gan, gyda and efo are usually used to denote possession or 'with', e.g.

Mae gen i radd C mewn bioleg.

Es i allan efo fy nghariad.

Oes newyddion gyda ti?

Cael

Cael is usually used when 'to have' means 'to obtain', 'to receive' or 'to get', e.g.

Bydd rhaid i fi gael tair gradd A.

Ydy e'n cael parti pen-blwydd?