In june / july, I'd say booking a day ahead will be enough. Or maybe a few days if you want to.
I last walked the via podiensis in summer 2022 and the gites I stayed at were sometimes almost empty.
One day I arrived after 8pm, there were only two other pilgrims in the gite. I called same day late afternoon to let them know I'll arrive late.
I mainly camped because I prefer that most days, but when I wanted to sleep inside, I always got a bed at the first place I tried.
If you want the demi pension (dinner and breakfast) then it is polite to make a reservation at least same day, or better the day before, though, so that your hosts can plan for how many people they need to buy food and cook.
If you're okay organizing your food yourself, then I wouldn't feel too bad arriving as a walk-in.