This is one of those times I'm gonna have to say "Yes" AND "No". Some Cancer men do genuinely enjoy gift giving to anyone they like; whether a co-worker, casual friendships, or deeper relationship; and often for Cancer's, giving their Time is a "gift" also. Though there are equally as many Cancer sun's, who aren't so keen on gift giving as a general personality trait, and whom only do so when they are speaking their "love language" & are showing they are interested in more than just casual friendship (which can happen subconsciously/unconsciously as well as intentionally). In order to have a better idea if your Cancer Guy is flirting or just being nice cause he likes to be nice, I would look at his Rising Sign, Venus Sign & South Node of the Moon signs; then I would compare how he treats women in his life whom are not his "type" romantically or whom are just friends that he's known a long time; if his rising, venus & south nodes are in social signs (like gemini, libra, etc...) andor he gives gifts to almost everybody he interacts with (regardless of gender), then it's likely he just enjoys gift giving and the good feeling that comes from making someone else's day (which is a big Cancerian drive), and likewise on the opposite side, if he doesn't have a lot of "social" or extroverted sign placements and he only seems to give gifts to women & only those who fit his "type", then he is more likely flirting with them.
I would also look at whether or not he's open, honest & direct with you when you ask him about why he gives gifts to these women (assuming you have asked him about it directly, calmly and with judgment already; which you should do if you haven't). When he's in a good mood and isn't hungry, try calmly and playfully asking something like, "Hey, I've noticed lately that you've been enjoying giving gifts to some of your friends, and it seems like you're mostly doing that with certain "lady" friends; have you noticed that pattern also? And are there any intimate feelings budding with any of them?", then observe his physical and verbal reactions; if he gets tense, gives you non-answers, doesn't start sharing his feelings, gets angry, suddenly needs to leave the room, says your just "reading into things", etc... then he's not standing in his truth in that moment, and likely feels ashamed that he has feelings for others & doesn't want to disappoint you or potentially face the consequences of deeper actions he's taken beyond gift giving.
Alternatively, if he tells you how it makes him feel (Cancer's motto is "I Feel"); like how it makes him feel good to give gifts to others, and why he felt like those individuals deserved a gift of the moment (maybe one or two had a bad day, or maybe one of them likes unicorns and he happened to see a unicorn cup when he was out buying groceries for dinner with you, or maybe one of them just got promoted, etc...). Gift giving on it's own, shouldn't be cause for concern, especially with a Cancer. It's gift-giving in combination with sneaky behaviors, heightened negative mood swings, evasive andor ambiguous communication tactics, or other indications of duplicity or agreed upon forms of infidelity/disloyalty, that indicate gift giving might be a valid red flag.