An Azure networking service that is used to provision private networks and optionally to connect to on-premises datacenters.
@Sabs ,
I would suggest to go with Hub and spoke architecture. Place your Gitlab in HUB VNET and peer HUB with all spoke VNET and connect HUB to On-Premises. By doing this the Gitlab can be accessed by On-Prem as well as spoke VNETs.