Routes configured with set static-route will not be installed in the "live" routing table used by the Gaia /Linux IP driver unless the nexthop address is DIRECTLY reachable from an active interface of the Gaia OS.
This difference between the Gaia "configured" routing table and the "live" routing table is covered in my Gaia 3.10 Immersion Video Series as it can be a little confusing to say the least, here is an excerpt from the course explaining this effect:

Attend my online "Be your Own TAC: Part Deux" CheckMates event
March 27th with sessions for both the EMEA and Americas time zones