I’ve been using version 2.5 for a little while now. I noticed that all forms seem to break the site layout at at a width between 390 and 420px. What I could find is that this grid layout, found in the native layout.css file is responsible for it.
It’s basically demanding 100% width, and then adding the column gap of 2 rem (which would be 32px by WP default setting) per gap.
.gform_fields {
width: 100%;
display: grid;
grid-template-columns: repeat( 8, 1fr );
grid-template-rows: repeat( auto-fill, auto );
grid-column-gap: 2rem;
grid-row-gap: 1rem;
}