Can you attach a picture of the visualization you're wanting?
If you're doing something like a table, just break it down to the highest level of detail. That way, each GPA value will have it's own line. You don't need to click 'don't summarize' that point and can apply conditional formatting. In the example I pasted below, each Month has it's own row in the data, so I can conditionally format the purchase values.
Here's a screenshot. trmgpa is the field in question. As you can see, if it's set to "Don't summarize", there is no "Conditional Formatting" option. If I choose any summarization, say Average for example, then the "Conditional Formatting" option becomes available.
Given this visual, I don't see why you would need to click 'Don't Summarize'. Set the [trmgpa] calculation to average or sum and then apply the conditional formatting. Is there a reason that didn't work for you?
I have no desire to summarize on the field. For one, I have records that have no value (null) for that field. For another, none of the summarizations are particularly useful in this context. I just want the value to be red if < 1.5.
Gotcha. Provided that this is the lowest level of detail though, your aggregate will be identical to your value.
For example, take the top row of your table:
If this represents a single row in your data source (let's say this is Bill's row), then Bill's GPA is 3.16. 3.16 is also the average of Bill's GPA and the sum of his GPA (since there is no other data taken into the calculation except for Bill's on this row).
Which takes me back to my initial remark; if you are at the lowest level of detail (Bill gets his own row and is not grouped in with anyone else), then you don't need to click 'not summarized' and can use conditional formatting.