Search is for people looking for something. GDN is to put your ads in front of someone that didnt ask anything. Its two different channels that don't need to be compared by ROAS. It doesn't make sense. From experience GDN is a lot of crap, bot and leftover of publisher deals. Work on whitelist you create, layer that with audiences, custom intent and interest. Exclude app's that burn through your budget. Use clickcease to avois fake clicks bot clicks as much as possible. And check your traffic quality in analytics.