Came across something interesting in an issue of the UK's (excellent) N64 Magazine recently - all the way back in August 1999 they dedicated a two-page feature to bootleg Game Boy stuff, pretty unprecedented for the time. they even gave it a...slightly familiar name, almost like a knockoff version of this very site (...except theirs was 13 years earlier):
(the quality isn't the best, sorry - I didn't have access to a scanner when I found this, so I just had to photograph it, but it's readable at least)
With it being 1999, unlicensed development hadn't really got into full swing, so all they had to cover were hacks and multicarts and the like; they seemingly didn't realise Mario 4 was a hack at all (which is fair enough, given the obscurity of the source game, Crayon Shin-Chan 4) and although they hint at Sonic 6 being based on some unnamed "poor game", they don't recognise it as Speedy Gonzales, and they really lay into it. Poor Speedy :(
They also cover Monster Go! Go! Go!, which they do identify as a Smurfs hack (the colour version of Smurfs being a fairly recent release at that point), but they don't provide any screenshots - my hunch is that they played Mario 4 and Sonic 6 through ROMs they downloaded from the internet, but Monster Go Go Go probably wasn't dumped at that point, so I'd guess they either actually obtained a copy (possible) or just nicked the boxart from a website and wrote up something based on the site's description (likely).
Amusingly, given their likely ROM-based Mario and Sonic shenanigans, they then proceed to get on their soapbox about the GB X-Changer (because playing pirated roms on your computer is just fine, but if you put them on a cartridge you have CROSSED A LINE SIR), and then there's a bit about some crap old multicart that someone on staff no doubt picked up on holiday, and that's your lot!