There are multiple places in Gopala Tapani Upanishad, Uttara Tapani where phrase gopalo'ham and so'ham occurs.
tasmād eva paro rajaseti so’ham ity avadhāryātmānaṁ gopālo’ham iti
bhāvayet, sa mokṣam aśnute | sa brahmatvam adhigacchati | sa
brahma-vid bhavati yo gopān jīvān vai ātmatvenāsṛṣṭi-paryantam ālāti
hy oṁ tat sad gopālo bhavati, yat so’haṁ || paraṁ brahma kṛṣṇātmako
nityānandaika-rūpaḥ so’ham || etad gopāla eva paraṁ satyam abādhitam
so’ham ity ātmānam ādāya manasaikyaṁ kuryāt || ātmānaṁ gopālo’ham iti
bhāvayet | sa evāvyakto’nanto nityo gopālaḥ ||
Therefore, in the consciousness that “I am the divine Lord beyond the
mode of passion,” one should meditate on the self, [thinking], “I am
Gopåla.” Such a person enjoys the liberated state. He attains the
status of Brahman. He becomes a knower of Brahman. He who from the
very beginning of creation takes the living entities, known as the
gopas, as his very self is verily Gopåla. One should think: The truth
is identical with the sacred syllable Om, and I am he. I am he who is
the Supreme Brahman, the essence of whom is Krishna, whose only form is
one of eternal bliss. That Gopåla is the unobstructed Supreme Truth.
Thinking, “I am he,” one should take control of the self and
concentrate the mind. One should then meditate on the self, thinking,
“I am Gopåla.” Verily Gopåla is unmanifest, infinite, and eternal.
gopālo’ham ajo nityaḥ pradyumno’haṁ sanātanaḥ | rāmo’ham aniruddho’ham ātmānam arcayed budhaḥ
I am Gopåla, unborn and eternal. I am the everlasting Pradyumna. I am
Råma and I am Aniruddha. The wise man thus worships the åtmå.
However these phrases are just one type of devotion and a state in which devotee identifies himself with object of devotion.
Baladeva Vidyabhusana explains these phrases in Govinda Bhasya, commentary on Brahma Sutras at Sutras 3.3.46 & 3.3.47. Explanation here.
In Bhagavata Purana too a similar phrase krsno'ham is seen:
gati-smita-prekṣaṇa-bhāṣaṇādiṣu priyāḥ priyasya pratirūḍha-mūrtayaḥ
asāv ahaṁ tv ity abalās tad-ātmikā nyavediṣuḥ kṛṣṇa-vihāra-vibhramāḥ
SB 10.30.3 — Because the beloved gopīs were absorbed in thoughts of
their beloved Kṛṣṇa, their bodies imitated His way of moving and
smiling, His way of beholding them, His speech and His other
distinctive features. Deeply immersed in thinking of Him and maddened
by remembering His pastimes, they declared to one another, “I am
kasyāñcit sva-bhujaṁ nyasya calanty āhāparā nanu kṛṣṇo ’haṁ paśyata
gatiṁ lalitām iti tan-manāḥ
SB 10.30.19 — Another gopī, her mind fixed on Kṛṣṇa, walked about with
her arm resting on the shoulder of a friend and declared, “I am Kṛṣṇa!
Just see how gracefully I move!”
This chapter of Vishnu Purana also has the words krsno'ham(5.13.25&26)
kṛṣṇe nibaddhahṛdayā idam ūcuḥ parasparam kṛṣṇo
'ham etat lalitaṃ vrajāmy ālokyatāṃ gatiḥ anyā bravīti kṛṣṇasya mama
duṣṭa kāliya tiṣṭhātra kṛṣṇo 'ham iti cāparā bāhum āsphoṭya kṛṣṇasya
Many of the Gopīs imitated the different actions of Kṛṣṇa, and in his
absence wandered through Vrindavan, representing his person. “I am
Kṛṣṇa,” cries one; “behold the elegance of my movements.” “I am
Kṛṣṇa,” exclaims another; “listen to my song.” “Vile Kālīya, stay! for
I am Kṛṣṇa,” is repeated by a third, slapping her arms in defiance. A
fourth calls out, “Herdsmen, fear nothing; be steady; the danger of
the storm is over, for, lo, I lift up Govarddhana for your shelter.”
And a fifth proclaims, “Now let the herds graze where they will, for I
have destroyed Dhenuka.”