Fortigate as gateway for multiple vpcs

Yes you can do that. Through Gateway Load balancer Endpoint in spoke + GWLB with Fortgiate as target in dedicated VPC.

See Diagram 2 in Palo Alto article with dotted green and dotted blue line flow ( forget about TGW in that diagram )

Essentially , you will created Gateway Endpoint in both spoke, link them with Gateway LB in dedicated VPC that has Fortigate registered with it.

After that some Appliances provide 2 ARM design where traffic will exit out to internet through second ENI of fortigate through IGW in fortigate/GWLB VPC. or in One Arm design where traffic will come back to spoke VPC and exit through spoke VPC IGW.

https://www.paloaltonetworks.com/blog/network-security/vm-series-integration-with-aws-gateway-loadbalancer/

I just want to give you architectural guidance that what you want to do is achievable.

Here you got.

https://docs.fortinet.com/document/fortigate-public-cloud/7.2.0/aws-administration-guide/570271/north-south-security-inspection-to-customer-vpc

Dont forget to accept the answer

Hi, why would you like to achieve this without a transit gateway? Technical requirement, cost? it depends on the reason the most suitable answer. If its because cost, then there other options like Gateway Load balancer but this also implies a cost, if it is because you would like to get other benefits like load balancing, north-south traffic inspection then gateway load balancer could be a solution without transit gateway. If is other reason and you just want to remove transit gateway, then before the transit gateway this kind of scenario was deployed using a Transit VPC, where one central VPC (your fortigate VPC) connects with every other VPC (spoke VPC) through a VPN connection. This architecture comes with own challenges, and transit gateway was the service that came to resolve them, however is still possible to do that configuration.