I'm going to break up with my girlfriend because I am no longer attracted to her due to he weight.
Has her weight gain turned you off?
Yes, I watch videos and the sight of her no longer turns me on sexually.
It's disrespectful to your girlfriend for judging her on her body weight. It makes you look shallow and others will in turn start judging you.
Then I'll just say she is no longer a good fuck.
It's very wrong to judge people based on their body weight, but regardless, it doesn't sound like you respect your girlfriend and you should end the relationship. I'd suggest you do so kindly and just tell her that it's not working out and you want to move on. Don't say anything mean that could hurt her feelings.