。
It rained so hard the field is a soggy mess.
Literal
Rain [subject-が] heavily fell [reason-ので] field [topic-は] soggy-mess [copula-だ].
ぐしゃぐしゃ is a mimetic for something thoroughly soaked and mushy — a muddy, waterlogged mess. It's more severe than じめじめ (damp) and implies the surface is ruined. グラウンド is borrowed from English 'ground' and specifically means an athletic field or playground in Japanese.