In my view, there is a subtle difference in meaning. Crucially, note that the adjective propinquus, which expresses a state, selects a dative (e.g., in propinquis urbi montibus (Nep. Han. 5.1)). So the dative nominal associated to the verb appropinquare, which is derived from the adjective propinquus, is used to express the final static position attained by ...
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: