Why is gastrin not acing paracrine manner primarily, when it acts mainly on cells nearby its release?
Paracrine signalling would mean not merely that gastrin is signalling the same organ, but that it is signalling nearby cells in the same organ - so that a few parietal cells close by respond, while the others perhaps would ignore the signal because maybe the paracrine signal would be degraded.  While the distinction is not absolute - there are shades of gray in all these things - blood gastrin tests are commonly done and medically relevant ( https://medlineplus.gov/ency/article/003697.htm ).  Because you can look at gastrin level as a whole-body characteristic, it is certainly appropriate to call it an endocrine signal.

Though their role is less emphasized, bear in mind that there are gastrin-producing G cells in the duodenum as well, and gastrin affects other digestive organs.