Gunman Clive HD Collection just recently saw release on the Switch, but it's actually been ready to go for quite some time. Developer Bertil Hörberg actually had the game up-and-running on Switch in about a week's time, and had it fully ready to go within the span of a month. Turns out that all happened back between Nov. and Dec. 2016.
Wondering why Gunman Clive HD Collection didn't release all the way back then? Hörberg says that a number of factors lead to holding the game back, including Nintendo's initial decision to hold back ports that didn't include new content. Some might remember that a handful of devs were struggling to get their games on Switch at first, and this was due to Nintendo blocking their release. Nintendo didn't want the eShop flooded with ports of games lacking new content right out of the gate, but obviously Nintendo has completely removed those restrictions now.