Translation:If Gareth will be revising Megan will be happy.
Because that's how you'd say it in English. Directly translated the Welsh is "If Greath will revise Megan will be happy." which doesn't make sense in English.
Not always, but they tend to, on the basis that if it isn't happening right now - in which case there would be no 'if' - then any uncertainty is in the future.
"If Gareth will be revising" is not present tense but future continuous and, as a native English speaker, surely this is an incorrect translation - unless it's used in the USA. I would say either "If Gareth revises..." or (possibly) "If Gareth'll revise=will revise". "will be revising" signals to me an L2 speaker.