Hi there, and

Gaslighting is a term that is tossed around in "pop psychology" these days. The actual description of gaslighting means to deliberately try to make another person question their reality in order to make them feel like they're going crazy. It is derived from the 1938 play "Gaslight" in which a woman's husband covertly tries to manipulate her psychologically by slowly turning down the gas lights and telling her they are not turned down so that she will think she is losing her mind.
If this is not your motive, then don't question yourself about that.
It is generally not recommended that you tell someone that you suspect that they have BPD. You can talk about behaviors, traits, etc. if that is something you are accustomed to doing with your partner, but generally we find that it does not go well when you tell a loved one that you think they may have BPD because they feel attacked and as if you are saying they are flawed.
What traits do you see that you think ring true with BPD?