if you're talking lambda then yes, you could use the same gauge (but not the same lambda values). a gas calibrated afr gauge will always show the afr's for gas, its not showing the true values for e85.
for example;
gas stoich = 1 lambda (14.7 afr)
gas moderate boost = 0.85 lambda (12.5afr)
e85 stoich = 1 lambda (9.76afr)
e85 moderate boost = 0.71 lambda (6.92afr)
what your afr gauge does is multiply the lambda values by its target fuel's stoich which is how we get our target afr's.
so on a gas afr gauge you arent seeing the 9.76 stoich of e85, you're seeing the 14.7 of gas (e85 stoich will always be 14.7afr on a gas wideband as you are still multiplying by 1, but that is where the similarity stops).
therefore, on a gas calibrated afr gauge you will multiply e85's lambda values by 14.7.
e85 moderate boost = 0.71 x 14.7 = 10.43afr on a gas calibrated wideband.