Not too busy when we visited on Tuesday evening.
If Wagamama's is new to you the style of dining may seem strange: the layout is informal, and mostly long tables with benches and stools so at busy times other people may be sitting close to you. Food comes to the table as soon as it is made rather than all arriving together.
Wagamama has a separate gluten free menu, and the manager both took my order and was at pains to make sure I understood that the work surface used to prepare my food would be cleaned down before preparation started, but that even so, they couldn't be absolutely certain of avoiding contamination etc. etc.
The GF menu had a limited choice but there was enough to not feel cheated. The GF dessert menu had 4 options, 3 of which were ice cream or sorbet. The 4th was a cookie dough / ice cream combination. But I was full already so didn't try this...