How to force GCP to use only one BGP path for outbound traffic over Partner Interconnect?

[NetNam-SGN]

Hi everyone,

I'm experiencing an issue with GCP and would appreciate your help:

• On GCP, I created two Cloud Routers, each associated with a Partner Interconnect VLAN attachment.
  Each Cloud Router establishes a BGP session with my on-premise router.
• From the on-premise side, I’m advertising the same route (192.168.90.0/24) to both Cloud Routers.
• As a result, in VPC > Routes, I see two routes for 192.168.90.0/24: one via Cloud-Router-1 and one via Cloud-Router-2.
• The problem is that GCP is load-balancing outbound traffic (from GCP to on-premise) across both BGP peers.

What I want is for GCP to prefer only one path for outbound traffic, and use the second one only as a backup (failover).

On the on-premise router, I’ve already tried adjusting MED, Local Preference, and AS Path Prepending. On the GCP side, I also applied BGP Route Policies to manipulate MED, but nothing seems to have any effect.

Can anyone help me understand how to solve this issue or force GCP to prioritize only one BGP path?