i'm looking for "blood: unlock the secrets guide" which was part of the original one whole unit blood released in 1998.
where can i find it? gog's version of one whole unit blood doesn't offer it.

GoG doesn't include it. Amazon lists three used copies for under $20 (as of 2018.05.23)

i don't want to pay extra for something which was included in one whole unit blood originally.
so i'm looking for a download location, where i can download this guide for free.

They probably couldn't secure publishing rights to the additional media. If you just want a walkthrough, the BloodWiki has a fairly decent one.

Not sure were you can find a copy of the text, but it looks like someone uploaded all the videos from the CD on YouTube

For anybody still interested, it's on the Internet Archive, i.e. here: https://archive.org/details/OneUnitWholeBloodUSA