We used an "ease of switching" factor in ours. Loyalty research suggests it's a moderating factor in customer loyalty, and it incorporates competition, risk and cost the customer must deal with to make a change. Use a scale, such as 1-5 or 1-7 where 1=no alternatives and very costly and risky to 7=easily switch in a matter of minutes for free.
Rather than go through the hassles of collecting and integrating data in Gainsight for an unproven metric, instead run an experiment and use statistical analysis to determine if the factor has an impact on the outcome. Use multiple regression for revenue and logistic regression for logo churn.