in part who pays the tax depends on what the trust instrument says and state law. it may allow the distribution of the capital gain to the beneficiaries. Allowable but not required. if it is distributed, then they pay the tax.
the trust may allow the trustee to distribute capital gains to the beneficiaries - then the question usually is which way produces the lowes taxes
on the other hand, even if the trust agreement is silent on such distributions state law may allow the trustee to use his discretion in deciding whether to distribute the gains. this becomes a legal question.
