The silver tube is called the gas tube...in airsoft it does nothing. On a real rifle it transfers gas back to the receiver to drive the bolt rearwards. If you want to keep your rifle looking realistic, then I'd keep it as part of the build.
The front of the handguards are held in place with the front handguard cap. It butts up against the front sight, within which the A/triangle front sight also serves to be the gas block. Typically when you remove the stock A front sight, you replace it with a gas block (which in airsoft doesn't actually do anything) and stick the gas tube into it. Without having something to but up against, the front end cap won't work.
Typically when you do what you're doing you'd replace the handguards with a free floating rail system. Some rails, such as the Daniel Defense Mk18, attach to a nut that replaces your delta ring (the part at the rear of the handguards that looks like a gear from a bicycle)...others, such as the CASV, attach to your delta ring and your flat top upper receiver.
However you could just use a different gas block (check airsoftparts.ca)...and butt the cap against that. Might look a bit wonky though.