Gaius Baltar seemed to hold a pseudo celebrity status, at least on Caprica amongst several of the more intellectual characters on Battlestar Galactica. As far as I can recall though, his major accomplishment was assisting in the design of the guidance systems used throughout the Colonial Fleet. How could this alone have given him enough name recognition to make Helo wiling to give up his own seat on the Raptor returning to Galactica? (which, don't get me started on how dumb of an idea that was!) Was he known for other great accomplishments?
Although he really was a genius and a working scientist, he was mainly known as a "Pop" scientist, rather like Carl Sagan, Isaac Asimov, Stephen Hawking, Albert Einstein... Come to think of it, this is a pretty common trope in our society, so it's probably common in the Colonies as well.
Technically, it's not clearly stated in-universe, but we see Baltar doing network TV interviews (and he's obviously comfortable in front of a camera so it's not his first interview), he's recognized by almost everyone who meets him, he seems to be "in" with both government and corporate entities, and he's certainly not hurting for money. No smoking gun, true, but a lot of circumstantial evidence points to his being a pop-culture scientist.
Despite his obvious character flaws, Gaius Baltar was known as the foremost expert in the fields of Computer Science and Artificial Intelligence. By extension, this also made him an expert on the Cylons. He was seen as eccentric or strange, but the public at large did not know how truly selfish & narcissistic he was.
Although best known for his design of the colony defense systems, we are led to believe that he had developed numerous other computer systems in use throughout the colonies as well. However, on the Galactica - which had no networked computer systems or AI - this knowledge was never truly showcased.
In today's world, Baltar would have enjoyed a status similar to Steve Jobs or Bill Gates - someone seen as a pioneer in the technology industry.
It's important to understand that Helo saved Baltar not only because of the scientist's status, but because Helo vastly underestimated his own worth to humanity. In the mind of Helo, he was just a soldier whereas Baltar was a great genius with something to contribute. Had he known the truth about Baltar, he probably would have shot him instead.