Definitely learn it as the hard G sound. I've never heard it pronounced more like an N, and I've been studying Japanese for 5 years! However now I'm curious which dialect it is pronounced more like an N...
It’s not an [n] but an [ŋ]. It’s common in the dialect spoken in and around Tokyo but is slowly disappearing. You can hear it in a lot of the Renshuu recordings, depending on who the speaker is.
It's not just about the が particle. In some phonological contexts, /g/ can be pronounced nasally with 鼻濁音 like ‘ng’ [ŋ]. It varies regionally and is said to be more common in dialects of Tōhoku region. It may sound similar to ん, but should normally stay distinct from clear /n/ onset as in な. It is always fine to pronounce it with a hard [g] and you should definitely learn to hear it as a variant of /g/ in contexts where it appears.
