Kinda depends on you. They're a bit fiddly to rebuild. I've resealed all kinds of springers and pumpers but the Diana 6M I resealed this spring was a bit more complicated. Here's a link to a post I made after the reseal:
This was certainly the most difficult airgun to reseal of any I’ve attempted. I purchased this non-working pistol on ebay for far too much but it was kinda pretty. Of course the big hope is that the internals hadn’t been destroyed by firing after the seals had gone or that someone hadn’t made a...
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That topic has some links you may find useful. Have a look at Danny Garvin's site which has the parts breakdown as well some instructions for both the rifle and the pistol. Nick Carter's site has the usual amazing step by step information about how he resealed a 6M. Once you read through that information you should have some idea of whether you want to tackle the job. It's pretty satisfying to put a non-working Giss back together and have it shoot but in some ways I was pretty lucky with mine. There is someone who will rebuild them for you but I can't remember the name, I'm sure someone will chime in with that.
In any case good luck with those, they're really quite amazing guns. Feel free to pm me if I can help any further.
Jay