This will be a first for the company, which has employed its previous devices with significantly slower modems. For instance, the OnePlus 5 and the contemporary 5T suport Cat.12 LTE connecitivity with downlink speeds of up to 600Mbps, whereas the previous-gen OnePlus 3 and 3T both had 300Mbps-capable Cat.6 LTE support.
Provided that your country and carrier have 4G LTE coverage with better than the average speeds, your OnePlus 6 will be better-suited to establish speede downlink and uplink connections to the network. Although a data throughput of 1Gbps is barely scraping the lowest possible minimum for 5G speed, which would theoretically be in the ballpark of 1-10Gbps, it still means super-fast LTE that will likely be a welcome improvement for owners of previous OnePlus devices.
And with the uncertainties that cloud the future rollout of 5G, gigabit-LTE is still the best we can get.