McClef is basically right. Welsh and Gaelic are not mutually intelligable. They ARE, however, related languages though, in that they belong to the same Celtic family. They bear the same similarities as English does to German or Dutch. Surprisingly, many Welsh people mistakenly refer to their own language as Gaelic. I'm not sure why this is. Perhaps some Welsh-speakers here could clarify this for us!

Here are some more word similarities:

Irish (Gaelic) Welsh English
dubh du black
glas glas green
Sasanach Saesneg English (person=literally mean "Saxon")
leabhar llyfr book
lamh llaw hand

So as you can see, there are similarities. While learning one does not allow you to understand the other, it would give you a head start learning it.
The three Gaelic tongues (Irish, Scottish Gaelic, and Manx are mutually intelligable - though Manx has a very different system of writing that the other two).

Is mise le meas,

Seán Liosliath Ó hAirt