nah they are completely different.
the gat has the english written parts, the maths/science questions & the humanities section.
while some parts of the latter two could possible be in the section 1 of umat, they aren't really similar, perhaps moreso the humanities part of the GAT as the umat section 1 makes it so you can't apply any outer knowledge to the situation, it's all about interpreting that data, and it's muuch harder then any part of the GAT.
also the other two parts of the umat are completely different with nonverbal reasoning and emotional reasoning.
and yes, the umat can be improved in all sections (particularly section 1 & 3) by doing heaps of practice questions and exposing yourself to different people/emotions (section 2), lots of different articles (Section 1) and IQ tests (section 3).
but the improving part is different for everyone, i've heard of people improving from below 50%ile to above 90%ile just by practice, and others haven't improved at all even with practice.