I posted earlier asking which bags to take on a United ERJ-145 flight and got lots of helpful advice.
On leg one, the Co-Pilot (very full with personal items and gifts) and the Synapse 25 (stuffed but not to the absolute max), fit very well. I did not receive a gate check tag. The Synapse fit easily in the overhead. It might have gone under the seat but I believe that would have been a tight fit, particularly due to the curvature on the plane side of the under seat area.
It helps that I had priority boarding due to my United credit card and got early access to the bin, which started to fill up.
I tried to attach some photos. Not sure how they will appear since I'm on my iPhone.