Vocabulary will increase gradually, so please check back frequently.

Literal

Words [topic-は] gradually increase [so-ので], frequently check please.

徐々に増えます ('will increase gradually') uses 徐々に (じょじょに, 'gradually'). こまめに ('frequently, diligently, at regular intervals') is a useful adverb for habitual checking. チェックしてください uses the loan 'check.' A standard website maintenance notice — 'we're adding content, keep checking.'