Hi,
In MMIC circuits I think that it's usual to provide a ground plane under the substrate. After they implant active devices,passives and interconnections they polish the backside of the substrate to make it thin,then they make vias to from side to side and add a metallization. Foundries like ommic,winfoundry,macom and other provide design kits basically for ADS with layout options for Cadence, so it's possible to simulate this things. If you don't have an GaAs kit or similar I suppose you can pick a GDS of a CMOS LNA made with CADENCE pick it with momentum and define a substrate with a backside metallization and add vias by yourself, so you can compare the behaviour of the same circuit with and without ground plane.
Hope it helps,