Galatians 2:7

ἀλλὰ τοὐναντίον, ἰδόντες ὅτι πεπίστευμαι τὸ εὐαγγέλιον τῆς ἀκροβυστίας, καθὼς Πέτρος τῆς περιτομῆς—

RBT Greek Interlinear:

Strongs 235; alla; ἀλλὰ; but; Conj

Strongs 5121; tounantion; τοὐναντίον; on the contrary; Adv-C

Strongs 3708; idontes; ἰδόντες; those who have perceived; V-APA-NMP

Strongs 3754; hoti; ὅτι; that; Conj

Strongs 4100; pepisteumai; πεπίστευμαι; I have been trusted with; V-RIM/P-1S

Strongs 3588; to; τὸ; the; Art-ANS

Strongs 2098; euangelion; εὐαγγέλιον; glad tidings; N-ANS

Strongs 3588; tēs; τῆς; the; Art-GFS

Strongs 203; akrobystias; ἀκροβυστίας; uncircumcised one; N-GFS

Strongs 2531; kathōs; καθὼς; just as; Adv

Strongs 4074; Petros; Πέτρος; Small Stone; N-NMS

Strongs 3588; tēs; τῆς; the; Art-GFS

Strongs 4061; peritomēs; περιτομῆς; circumcision; N-GFS

RBT Translation:

ἀκροβυστία akro-bystia - "Crest depth"

But on the other hand, those who have perceived that I have been trusted with the Gospel of the End Depth,³ just as Small Stone, the Cutting Around.

The Crest of the Depth vs. the Cutting Around "Circumcision."

LITV Translation:
but on the contrary, seeing that I have been entrusted with the gospel of the uncircumcision, even as Peter to the circumcision,

ESV Translation:
On the contrary, when they saw that I had been entrusted with the gospel to the uncircumcised, just as Peter had been entrusted with the gospel to the circumcised

Footnotes

3

The Akro-Bystia

Strong's #G203 ἀκροβυστία akro-bystia. "head depth, peak depth, deep point" This is a word modified from the normal Classical Greek ἀκροποσθία akro-posthia (tip of foreskin) because posthia/peos (ποσθία/πέος) referred to the male penis and/or foreskin. The first part ἀκρο "akro" means "end, farthest point, tip, limb, extremity, rim, crest." It also can mean "citadel built on a steep rock overhanging a town."

• headland, cape, Il. 4.425, 14.36, Od. 9.285, S. Tr. 788, Pl. Criti. 111a: metaph., ἄκρην πενίης οὐχ ὑπερεδράμομεν Thgn. 619, cf. A. Eu. 562; κάμπτειν Men. 4.
• hill-top, height, Od. 8.508, Hymn.Is. 72 (pl.). 
• a mountain-top, summit: used by Hom. only in the phrase κατʼ ἄκρης from top to bottom, i. e. utterly, πόλιν αἱρέειν κατʼ ἄκρης Hdt.; so in Attic, κατʼ ἄκρας utterly, Trag., Plat. 
• the citadel of a city, Lat. arx, Xen. 

Absent any signification in this compound for the male "membrum virile" we are left with βυστία bystia. What is this? No one seems to know, and the assumption was that it came from a different pronunciation of "posthia." There does seem to be a word play here pos-thia vs. bys-tia.

The prefix βύσ- in Greek is often associated with depth or something deep. This understanding helps clarify the meaning of βύστος/βυστία in this context. It is the prefix that forms "Abyss" ἄβυσσος (ábyssos) usually translated as "the Deep."

The Diccionario Griego-Español (DGE) provides a brief and uncertain meaning of the word βύστος (bystos), suggesting that it might mean "submerged in the depths" and notes its association with the term "καμπτός" (bent, curved) and its heavy-sounding nature, as explained in the context of the name "Ἥφαιστος" (Hephaestus) (cf. DGE βύστος).

Strongs #G899 βάθος is a word meaning depth and is used in either direction: depth, height — (accusative, as measured down or up) as in "deep into the mountain" and hence,

Ω (omega) depth of the riches! Also of wisdom and knowledge... (Romans 11:33 RBT)

the deep ones of the God. (1 Corinthians 2:10 RBT)

If the author were really after "uncircumcision" he could easily have used ἀπεριτομῆς "without circumcision/cutting around."