I want to create my own SMS gateway for my business through a PC.
Jan F, such a comprehensive response! That's great!
Unfortunately, I just can talk about SMS gateways, since I got into connect with Ozeki's SMS gateway a week ago. Now I use its free trial for messaging. So, if your purpose is sending/receiving SMS messages to/from your partners I honestly recommend it for anybody to try out this software.
It can be used for 2-way SMS/MMS communication. For SMS messaging you need to connect your SMS gateway to the mobile network through SMPP IP or GSM modem connectivity. This guide can help you to choose the best connection method: http://ozekisms.com/index.php?owpn=647
Otherwise, this SMS gateway provides many APIs for developers, so if you need a special/unique solution, I think it is worth to check this site: http://ozekisms.com/index.php?owpn=95&info=product-manual/developers-guide
First of all this will (probably) only work on GSM networks - at least I'm not aware on setting one up yourself on a CDMA network.
You'll need an activated SIM card, a GSM modem, a computer and a SMS gateway software. One might be able to set it up using UNIX and open source software but that's out of my knowledge.
As for the gateway software here are some links - I haven't tested any of them:
Generally I don't see any benefit in setting one up for your own business. Unless your entire staff only get's those old Nokia or Samsung phones that only support WAP there is no point in using SMS instead of emails in the first place.
SMS gateways are one of those things that became known years past their necessity. You can get a $0 smartphone that does internet and email with every mobile contract you finalize.
If you are just looking for an option to send out SMS from a computer I suggest looking into one of the many online SMS services out there.
If you are looking for an option to send out SMS in bulk e.g. as newsletter an SMS gateway is the wrong choice as it's designed for one-to-one communication between SMS and email. You should be looking into bulk or marketing SMS services.