Where are you seeing it advertised? "GSM unlocked" most likely means it's a GSM phone (as opposed to a CDMA phone -- CDMA was previously used by Verizon and Sprint, but has since been shut down), and that it's carrier-unlocked. So it will probably meet your needs, assuming the seller is being completely honest.
Thanks for your response. That was what I was looking for. I recently purchased a S9+ and was concerned about it being unlocked since I have a sims from my previous phone and wanted to use it again. I understand that CDMA does not use sims but the GSM does and you can use sims chips from another phone. I did this before but wasn't sure this time.
CDMA phones in later years did use SIM cards as well, primarily for the 4G signal (CDMA is a 2G technology). The best way to know for sure is to get the exact model number of the phone, so that you can look up the specs.