Add this code in custom.scss file or Gantry Page Settings:
.g-infolist .g-infolist-item-img.avatar img {
border: none;
}
To add custom style sheet to Gantry 5 Framework please follow these instructions:
docs.gantry.org/gantry5/tutorials/adding-a-custom-style-sheet
or if you want to add them in the Page Settings:
docs.gantry.org/gantry5/configure/page-settings#custom-css-js