Terminology question: gas giant vs gas planet While not exactly the most exciting question, I'm wondering: is there any real, semantic difference between a gas planet and gas giant, or are the two terms used interchangeably by most in popular usage (i.e. popular-scientific texts)? 

now, a gas planet is obviously the umbrella term for all planets composed mainly of gas, but I often see "gas planet" and "gas giant" used interchangeably without any real change in meaning. I'd class Neptune &amp; Uranus as "ice giants" and Jupiter &amp; Saturnus as "gas giants", but (popular) dictionaries like https://www.thefreedictionary.com/ ; https://en.oxforddictionaries.com/ still seem to group them all into gas giants.
"Gas giant" is the most standard term for planets in the Jupiter/Saturn class, and, as you said, "ice giant" (since the 90's) is the term for planets with compositions like Uranus and Neptune. 

"Gas planet", on the other hand, is just not a commonly used term. Judging by Google n-grams, it is less than 1/12 as frequent. Also, if you do a Google book search, of the times it is used, it is very often as a part of the phrase "giant gas planet." I would consider "gas planet" simply to be a description rather than a technical term, at this point.

A lot of likely materials for Earth-sized non-rocky planets would be more likely to be classified as ices in the outer solar system. So planets that deserve to be called "gas planets but not gas giants" may be quite rare, so it's possible a distinction may not be needed.