spi communication with gsm module

I would try and aviod doing it. Try to find a GSM module or system with a sensible interface which will do what you want.
Last time I tried to set up a GSM module it was a nightmare...when it was finally done the customer rejected it as 'too user hostile' and I ageed with him.
Del

GSM module with RS232 either RS485/422 interface would be considered as likely suitable for your application.

can we control with micro controller without using anything inbetween

It depends upon what exactly is structure of your project.

For instance it could be look as follows

PLC-->GSM_module-->...remote distance site...GSM_Module->RS485_Network

either

PLC(uC)-->GSM_module->controlled device(sensor)

It's not clear what you meant under "without using anthng beetwen"?

Between PLC and GSM should be placed some interface.

we want to use uc to control gsm module directly to tx and rx sms (text message) only. anything inbetween means no rs232/485 device between uc and module.

Ok, it's quite possible. I'm supposing your uC [of course properly programmed] is capable to handle RX/TX serial data.

we can select the uc as per the requirement but we prefer if we can use a uc without uart. first preference is to use the I/O in synchronous mode, second preference will be to use spi communication. uc can be programmed if we know the timings / logics of the modules. commercialy available module work with at&t commands

I can't get why don't you want use and uart? To my opinion GSM modules more suitable for using for non-so-crucial real time applications where asyncronous uart mode is quite enough. I'm wondering why you prefered syncronous mode.

I'm planning to use GSM in one my project, but only for informing me and customer about how is good/bad equipment is operating. I wouldn't rely to send any "turn on/off" commands through GSM due to a great risk that something could going wrong by reasons that i can't resolve in any way (damn mobile operator, lack of SIM-card credit and so on).

It's only my point of view. If you have any arguable reasons I do believe you'll do what you conceived.

One more buzz. I do supposing you're aware that any protocol is composed of two parts -- electrical (physical) and software. I would agree with Del reasons here, wheter you want spent your time dealing with spi to RS232/485 plausible troubles?

The GSM module I used had an I/O pin which could be triggered by an external event. The GSM module could be programmed by plugging in a PC or laptop and typing a sting of unecessarilly complex command strings. It did work but the user interface was pathetic.
It was an Enfora gsm module from this company. I have seen one (but I can't remember who made it) with a nice GUI interface which allowed you to type the phone numbers and text messages in via your laptop. It even allowed you to call multiple numbers and set priorities for what order to ring them if there was no response.

Del

This thread which I stared a while back about the problems I encountered may be of interest.

Good luck
Del