With "Ab Gallia venio. or "De Gallia venio". (Or other sentences of the same kind)
Could it mean both, I come from this place, or I am originated from this place (like a nationality, or the origin of my family)?
In Classical Latin:
- ā Galliā veniō means "I'm coming away/going away from Gaul" or "From Gaul I'm coming to.."
- dē Galliā veniō means "I'm arriving from Gaul" or "After/From Gaul I'm coming to.."
- you can also express this more naturally with dē/ā Gallīs veniō
The use of venīre in the sense you're after doesn't seem to be listed in OLD. Instead you can use:
- ortū sum Gallus
- ortus sum ā/ē Gallīs
- oriundus sum (ā/ē) Gallīs (acc. to Smith&Hall expressing a more remote origin, e.g. of the family's descent)
- in Galliā nātus sum
- ā/dē Gallīs, dē Galliā orīginem dūcō/trahō/habeō
