To make the "#¤" thing work in a semi-programatic fashion, after multiple trials and errors - we either need direct interpolation or to do the following (which worked for me):

frontendIPConfigurations: [{
            name: "appGwPublicFrontendIp",
            publicIPAddress: {
                id: pubIp.id
            }
        }],
        frontendPorts: appGw.frontendPorts,
        gatewayIPConfigurations: appGwIPC,
        httpListeners: [{
            frontendIPConfiguration: {
                id: rg.id.apply(id => `${id}/providers/Microsoft.Network/applicationGateways/${rgInfo.nodeName}${appGw.suffix}/frontendIPConfigurations/appGwPublicFrontendIp`),
            },
            frontendPort: {
                id: rg.id.apply(id => `${id}/providers/Microsoft.Network/applicationGateways/${rgInfo.nodeName}${appGw.suffix}/frontendPorts/frontendPortName`),
                
            },
            name: appGw.httpListeners[0].name,
            protocol: "Http",
        }],

(where

rg

is my resourceGroup defenition,

rgInfo.name + appGw.suffix

is the name of the appGw as defined on top)