Merchants are greyed out when the value being redeemed is not offered by the merchant. Some merchants only offer specific values e.g. $10, $20, $100 etc., while other merchants accept a range of values e.g. $10-$500. These are listed below the merchant name as you browse.
To access greyed out merchants, gift recipients could simply split their Guusto gift into multiple merchant gift cards (e.g. a $75 Guusto gift, could be split into 1x $50 Starbucks gift card and 1x $25 Best Buy gift card).