First ones that comes to mind are Fill Me Up and Gain of Life, and while it’s not a visual novel Virtual Gainer 2 is close enough and revolves entirely around the player’s gain.
There’s probably a way to search by multiple tags at the same time (visual-novel & protag-wg) but I couldn’t get it to search for only topics with both tags instead of either. You could also check out Krod’s weight gain games recommendation list for some others that I’m no doubt forgetting, there were a lot of visual novels on there last time I looked.