So I have a gf. She's 19, I'm 23. But I'm totally into older women. Like much older, like 40s. First, is this normal? and second, how do I tell my gf about this?

Age ain't nothing but a number. This is normal. The way to tell your gf is to just do it. Do it now before she gets too deeply involved. Do yourself a favor and tell her now so you can pursue older women. They like younger men because we can satisfy their sexual desire!

OKAY, YOU ARE IN A RELATIONSHIP WITH THE 19 YO, SO YOU MUST SEE SOMETHING IN THE 19 YO RIGHT? YOU CAN TELL HER THAT YOU USUALLY DATE OLDER WOMEN AND THAT SHE IS YOUR 1ST TRY WITH A YOUNGER WOMAN (IF THAT'S TRUE). THE QUALITIES THAT YOU LIKE IN OLDER WOMEN MAY BE A QUALITY THAT SHE POSSESS... LOOK WITHIN YOUR HEART TO SEE WHAT YOU REALLY WANT. SOMETIMES WE DO NOT END UP WITH THE TYPE OF PERSON THAT WE 1ST THOUGHT WE WERE ATTRACTED TO. ALSO, KEEP IN MIND THAT SOME OLDER WOMEN LIKE YOUNGER MEN FOR DIFFERENT REASONS. MAKE SURE THAT YOU ARE NOT BEING USED FOR SEX. IT MAY START OFF FUN, BUT FEELINGS COULD BE HURT. GOOD LUCK!

First of all be honest to your girlfriend. Honesty is the key to any relationship. Then go for it. If you want an older woman that is your choice but first you need to get your priorities in order. Is your girlfriend someone that you are willing to loose to look for this older woman?
Think first then react second.