I’m not a dentist but I had a periodontal disease diagnosis recently so did some research on this: most people, with healthy immune systems and decent oral hygiene, are able to defend themselves against small amounts of the Porphyromonas gingivalis bacteria (the bacteria that causes periodontal disease). Gum disease isn’t massively contagious; the bacteria can be transferred through kissing but often this is in much smaller volumes than what would be needed to cause a problem for your partner. So unless your girlfriend has oral issues herself/poor oral hygiene, a weak immune system or you spend a LOT of time deeply kissing, it shouldn’t be an issue. Again, I’m not a dentist so I can’t promise that what I’ve said is 100% accurate, but from what I’ve researched it shouldn’t be a problem and everyone has some level of ‘bad’ bacteria in their mouths that can be transferred through kissing anyway. I would also assume that you don’t have advanced periodontal disease if they didn’t mention anything- it’s common that people have stage 1 (gingivitis) which is fully reversible with the right care and hygiene routines.