I am running a GURPS game set in Mike Pondsmith's Cyberpunk.

One of my players has been asking endless questions about the nature of the tech in the setting. "Does wireless hacking exist? What are the limitations of cyberware? What about smart clothes? Can I get a drone? What about smart grenades? Can monowire cut through walls? Why not? That doesn't make sense because realistic \[thing\]. That's kind of retro-futurist though and I don't like that. What about this thing? How about this?"

Sometimes I have an answer, sometimes I don't. I feel pressured to deliver an answer but sometimes I need to consider the gameplay ramifications of such an answer.

I like this player. How can I gently shut down discussion on things that I can't feasibly answer immediately but the player insists are important to know right now?

You tell them to either look it up on their own time in the rules or wait until later when you have time to deal with it. It’s rude to use you as TTRPG Google when you’re in a session. They can stop being lazy and look for themselves or just hold their questions.

Just talk to him about it. Explain that you don't have the knowledge to provide a perfect answer and work with him to understand why he wants to know that. What does he hope to accomplish. Once you know what the player's idea is, you can work together to figure out how you can go about it. Either enabling it or telling him you're worried about how that might impact the game in the future if it becomes the norm.

Usually happens prior to a session!

If it doesn't break the game, I sometimes ask my players what they think they can have access to. If they tell me what and why, I usually permit it.

Man, that sounds exhausting. Is your player remembering that this is a game of make-believe? I like to subtly make that point now and again. GMs have neither the time, energy or brain power to have decided everything and its ramifications up-front. If they have pertinent questions that's one thing but just trying to examine the world as if the GM is its Wikipedia is not fun.

tell them you don't have the answer right now, but if they write it down you'll rule on it before the next session. This, I find, works for rules conundrums and setting elements alike. Players have been disappointed the first time when I haven't been able to answer everything off the cuff, but they realize I'm only human and can't know every detail of an imaginary setting.

I have a player like this, but he messages me heavily over the weekend after sessions. I just mute the conversation until I feel like answering. Sometimes I give full answers, sometimes I am very short and dismissive. The last time I simply said that was a question for Google not for me. I am not going to be bothered about whether a subclass exists, stop being lazy or don’t have an answer, not my problem.

You don’t have to answer. Your time is valuable. You do not owe anything to anyone. Answer what you want when you want and to the extent you feel like it.

It sounds like he's asking because he's taking genuine interest in the world and wants to better understand his place in it. I would see this as a good thing.

Roleplaying is collaborative, it goes both ways. It isn't just a GM telling a story.

If you don't have these answers, ask if he would like to help you figure them out. If you don't know how smart clothes would work, ask how he thinks they would be implemented.

I remember having a lot of fun when my players would have good input and add details to the world that became part of it. It helps keep both of you creative and adaptive.

I don't run GURPS but I plan to run a sci fi game soon and I've talked to my players about the general atmosphere I'd like the game to have, including that feel of retrofuture and soft sci fi where things don't really need to make sense... precisely because I ain't got time or interest for all this hard sci stuff xD Also ngl I'd rather the vision of the future the 80s captured than extrapolating futuristic possibilities from the current year, where 80 AI agent-steered cameras are on you at all times and if they're not, at least 3 different devices are listening in on your conversations. Giving them that spiel in session 0 helped them get what kinda game I wanted to run and I think might have shut down some more inquisitive players' testing. If you haven't had that sorta talk and there's more than one player whose behavior you'd like to see change, you could come in for a second session 0 but again and bring that up there, while you're at it might as well ask 'em what they think is going well and what could be changed and stuff.

For best results just be honest that you are only able to keep up with so much while running the and tell him that after a certain point you need to have some questions be asked between sessions if he wants answers by the time a session starts. In a world as complicated as cyberpunk there's only so much any one human can keep up with anyway. If he's that invested and you want to build trust you can also encourage him to read the rules and lore and greenlight anything he can provide a page citation for. Alternatively for less good results but still effective you can make a point of bad guys using the same fancy tech he asks about. If the party uses diy drones, the corpos can provably afford to whip out high end halflife-2 ass buzzer killbots, etc.

I'm reading this as enthusiasm and interest in the game and setting?

I think if you don't have an instant yes/no, just say you need some time to consider the mechanical impact of whatever they're asking about.

Would 300% rather deal with this than a silent scroller who waits to be prompted to pick up the dice.

"What do you want to do with that?"

I find it very useful to talk about intent in these cases. Some players like to play game where they try to trick the GM into something with dozens of questions, that as a whole leads to them breaking the game.

And some players just don't know that they can just tell you what their actual goal is, and then work with you on that.

When talking it over, two questions worth asking is, "What are you trying to do?" That way you know what their goal is and keep that in mind. And also, "How would you feel if more powerful people than you could do that?"

In Shadowrun, something like wireless hacking can be super powerful. Players absolutely get this idea in their head they can brick the enemy's guns. And there are long and involved rules for it and everything. I just don't use those rules because players can also agree it's not fun getting your own equipment bricked.

Well, that's at least a good sign.