What is the great achievement by this maharishi? How the God appeared before him and what the God asked him to do?
Gunaseela MahAtmiyam(குணசீல மஹாத்மியம்)
Gunaseela Maharishi is the devout sishyA of ThAlpiya Maharishi(தால்பிய மஹரிஷி). Once ThAlpiya Maharishi along with his sishyAs went to the Himalayas to worship Vyasa then took Ganga snAnam and snAnam in other holy waters. On their return, they stopped at ThiruVenkatamalai (திருவேங்கடமலை) to worship ThiruVEnkatamUdayAn(திருவேங்கடமுடையான்) or Perumaal Venkateswara.
Gunaseela Maharishi felt inseparable devotion to Perumaal and beseeched Perumaal to come with them. On hearing this devout Bhakta of his, Perumaal said that he had committed to grace darshan onto devotees from Thirumalai till the end of the current Kalpa and told Gunaseela Maharishi to perform Tapasya upon returning to the Ashram, by receiving the right notion from his GuruNatha ThAlpiyar to undertake the same, assuring Gunaseelar that his intention will be duly fulfilled.
Gunaseelar then told his GuruNathar about this, to which ThAlpiyar responded, "It is well known that future events are foretold in the Puranams. In that way it is known that as a result of your Tapasya, SriVaikunthaVaasudeva will rise in your Ashram located in the North bank of Cauvery as DivyaMangalaVigrahaSwaroopan with ThiruNaamam of Sri Prasanna Venkatesan(ஸ்ரீ பிரஸன்ன வேங்கடேசன்) along with Sri Devi ThAyAr gracing Devotees with their presence and granting the bhaktas' wishes till the end of the current Kalpa"(ஸ்ரீதேவியுடன் ஸ்ரீ பிரஸன்ன வேங்கடேசன் என்ற திருநாமத்துடன் இந்த கல்பம் முடியும் வரை திவ்யமங்கள விக்ரஹ ஸ்வருபனாய் எழுந்தளியிருந்து). Having said so, ThAlpiya Maharishi blessed Gunaseelar.
Gunaseela Maharishi upon returning to his Ashram, after duly bathing in the river Cauvery, undertook steadfast Tapasya, meditating with his Manas one-pointed on SriVaikunthaVaasudevar(ஸ்ரீ வைகுண்ட வாஸுதேவனை மனதில் தியானம் செய்து தவம் இயற்றலானர்). His Tapasya undeterrable - amidst PanchAgni in the summer, donning wet vasthram in Winter, Standing in Water during Monsoon. Devas became aware of it and became afraid. Indra, fearing that his post might be usurped, launched a host of attacks with Thunder, Rain, Storm, Lightning and other numerous means attempted to shatter Gunaseelar's Tapasya in vain.
The Devas flabbergasted by defeat approached Lord Brahma and enquired onto the reason for Gunaseelar's Tapasya. Brahma immediately saw with the power of his Jnana Dhrishti that Gunaseelar, with the express intent of being graced by Perumaal in his Ashram, was meditating unshaken on SriVaikunthaVaasudeva. Thus, having heard from Brahma that they had nothing to fear, the Devas paid respect to Lord Brahma and returned to their abode. Such is the greatness of Gunaseela Maharishi's meditation on Perumaal.
In due time, EmPerumaal SriVaikunthaVaasudevar along with SriLakshmi ThAyAr set out on VedaSwaRoopan GarudAzhwan to Gunaseela Maharishi's Ashram to grace him who had taken Perumaal himself as his only refuge and meditated unwaveringly.
Knowing thus, that Perumaal and ThAyAr are arriving from Vaikuntham, Brahma & Saraswati, Siva & Parvati, Devis & Devas, Sanatkumara Munis, Sage Naradar, Thumburu & GandharvAs, Kinnargal & Kimpurushargal, Yakshas, Sakhyas, SAranAs & others and ThAlpiyar & his Sishyas arrived to Gunaseela Maharishi's Ashram and awaited eagerly Perumaal & ThAyAr's Arrival.
On Arrival, EmPerumaal grants glorious Darshan to Gunaseelar(which is described in exquisite detail here) and grants Maharishi Gunaseelar a boon - through which he asks for SrimanNarayana to grace all his devotees and grant their wishes by taking Nitya Vaasam at his Ashram till the end of the Kalpa. Perumaal obliges Gunaseelar's wishes as Sri Prassana Venkatesan carrying Lakshmi ThAyAr in his chest appearing with Shanka, Chakra, VaradaHastham & KadiHastham(கலியுக தெய்வமாய்த் திகழ ஸ்ரீ பிரஸன்ன வேங்கடேசனாக திவ்யமங்கள விக்ரஹஸ்வரூபனாய் சங்கு சக்கரம் வரதஹஸ்தம் கடிஹஸ்தம் உடையவராய், திருமார்பில் இலக்குமியைத் தாங்கி நின்ற திருகோலத்தில் காட்சியளித்தார்).
Source: Official Temple site