I've been playing guitar for 1 and a half years, and I would say that I'm OK, not great on improvision. I can improvise over chords with the pentatonics, major, and minor scales. The other day, I listened to Marooned by Pink Floyd, and I was wondering how I could improvise as awesome as that, because David Gilmour said that this song was mostly improvision.
First, you need to realize that you're not David Gilmour. There's a reason he sounds the way he does. Can you guess that reason?

I'm really not trying to be mean here, but you need to stop trying to sound like someone else and sound like yourself.

There's nothing wrong with borrowing bits and pieces from your favorite artists - we all do it. However, you need to develop your own style. Ideally, your playing style should be all you, with hints of those you like.

Be original. Be you.

Well first of all you need to practice. I've been playing guitar for about 9 years now, and I'm pretty good at improv, but I'm still working on it.

Second, what ^that guy said.
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Master your scales ( pentatonic in Gilmour's case ) and practise fretboard visualization when you are not physically playing ( it is something I neglected for almost 15 years ) also when your ready for the next step start learning the modes and how they interact within music. It is very difficult to improvise when you don't know what notes are relevant. Don't forget their are also accidentals ( in chords for example If you had a Dmajor chord with a D# in the root it would be noted as D#/D I believe) but they are more prevalent in jazz.
You can't "improvise like someone", that's a silly notion. You can copy a player by playing their music, or you can improvise which is by definition YOU creating something on the spot using YOUR knowledge and YOUR experience of music. If you're consciously copying someone else then you're not improvising.

By all means learn about someone's playing style - learn their solos, learn how they construct them, learn about their phrasing and the theory behind them. Understanding what they do will help you recycle their ideas and use them in your own playing but you're still not going to be copying another player and you won't sound like them, you'll still sound like you.
Actually called Mark!

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Pick 4-5 solos by DG and try to identify features that are common to them. Note choice, vibrato speed & depth, how much he uses space in his phrasing, etc. Treat it like a mini project. Transcribe licks that you like and try to work some of it into your playing.

There's certainly nothing wrong with learning from players you admire but do draw on other resources and inspirations too.
You need to get your bends and vibrato down, Dave was a master of big as smooth as butter bends and mastering your control over the very subtle finer points of playing is what you want to aim for.

Learning your scales up and down the fretboard and becoming comfortable is one thing, but that's a given, the real deal is training yourself to be able to make the most out of every note. 1.5 years playing is early days, just take it step by step and make sure you're absolutely 100% aware of the smaller finer details going on.

Ultimately you can attempt to copy his style but you will never be able to really improvise like him, learn from him and steal ideas but do your own thing, that's how the best guitarists are created.
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Transcribe his stuff and work on your left hand legato (hammers/pulls, bends, slides).
Get very comfortable with the minor pentatonic scale all over the neck, and do LOTS of ear training, should set you on the right track!
Take your time, think about space between phrases, big 2-3 step bends, minor pentatonic licks, big wide vibrato, but most of all try and nurture your own style. If you become famous you dont want to be known as Gilmour Mk2....